Add custom decoder inits for various oci types that use omitempty (#347)

Many fields on the various OCI types use "omitempty" for encoding and
decoding the json representation in golang. This PR adds custom json
decoder functions to allow for behavior similar to "omitempty".

---------

Signed-off-by: Kathryn Baldauf <k_baldauf@apple.com>

Sources/ContainerizationOCI/Spec.swift

@@ -61,6 +61,27 @@ public struct Spec: Codable, Sendable {
         case root
         case linux
     }
+
+    public init(from decoder: Decoder) throws {
+        self.init()
+
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        self.version = try container.decode(String.self, forKey: .version)
+        self.hooks = try container.decodeIfPresent(Hook.self, forKey: .hooks)
+        self.process = try container.decodeIfPresent(Process.self, forKey: .process)
+        if let hostname = try container.decodeIfPresent(String.self, forKey: .hostname) {
+            self.hostname = hostname
+        }
+        if let domainname = try container.decodeIfPresent(String.self, forKey: .domainname) {
+            self.domainname = domainname
+        }
+        if let mounts = try container.decodeIfPresent([Mount].self, forKey: .mounts) {
+            self.mounts = mounts
+        }
+        self.annotations = try container.decodeIfPresent([String: String].self, forKey: .annotations)
+        self.root = try container.decodeIfPresent(Root.self, forKey: .root)
+        self.linux = try container.decodeIfPresent(Linux.self, forKey: .linux)
+    }
 }
 
 public struct Process: Codable, Sendable {
@@ -78,6 +99,22 @@ public struct Process: Codable, Sendable {
     public var args: [String]
     public var terminal: Bool
 
+    public enum CodingKeys: String, CodingKey {
+        case cwd
+        case env
+        case consoleSize
+        case selinuxLabel
+        case noNewPrivileges
+        case commandLine
+        case oomScoreAdj
+        case capabilities
+        case apparmorProfile
+        case user
+        case rlimits
+        case args
+        case terminal
+    }
+
     public init(
         args: [String] = [],
         cwd: String = "/",
@@ -120,6 +157,40 @@ public struct Process: Codable, Sendable {
         }()
         self.init(args: args, cwd: cwd, env: env, user: user)
     }
+    public init(from decoder: Decoder) throws {
+        self.init()
+
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        self.cwd = try container.decode(String.self, forKey: .cwd)
+        if let env = try container.decodeIfPresent([String].self, forKey: .env) {
+            self.env = env
+        }
+        self.consoleSize = try container.decodeIfPresent(Box.self, forKey: .consoleSize)
+        if let selinuxLabel = try container.decodeIfPresent(String.self, forKey: .selinuxLabel) {
+            self.selinuxLabel = selinuxLabel
+        }
+        if let noNewPrivileges = try container.decodeIfPresent(Bool.self, forKey: .noNewPrivileges) {
+            self.noNewPrivileges = noNewPrivileges
+        }
+        if let commandLine = try container.decodeIfPresent(String.self, forKey: .commandLine) {
+            self.commandLine = commandLine
+        }
+        self.oomScoreAdj = try container.decodeIfPresent(Int.self, forKey: .oomScoreAdj)
+        self.capabilities = try container.decodeIfPresent(LinuxCapabilities.self, forKey: .capabilities)
+        if let apparmorProfile = try container.decodeIfPresent(String.self, forKey: .apparmorProfile) {
+            self.apparmorProfile = apparmorProfile
+        }
+        self.user = try container.decode(User.self, forKey: .user)
+        if let rlimits = try container.decodeIfPresent([POSIXRlimit].self, forKey: .rlimits) {
+            self.rlimits = rlimits
+        }
+        if let args = try container.decodeIfPresent([String].self, forKey: .args) {
+            self.args = args
+        }
+        if let terminal = try container.decodeIfPresent(Bool.self, forKey: .terminal) {
+            self.terminal = terminal
+        }
+    }
 }
 
 public struct LinuxCapabilities: Codable, Sendable {
@@ -160,6 +231,14 @@ public struct User: Codable, Sendable {
     public var additionalGids: [UInt32]
     public var username: String
 
+    public enum CodingKeys: String, CodingKey {
+        case uid
+        case gid
+        case umask
+        case additionalGids
+        case username
+    }
+
     public init(
         uid: UInt32 = 0,
         gid: UInt32 = 0,
@@ -173,16 +252,42 @@ public struct User: Codable, Sendable {
         self.additionalGids = additionalGids
         self.username = username
     }
+
+    public init(from decoder: Decoder) throws {
+        self.init()
+
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        self.uid = try container.decode(UInt32.self, forKey: .uid)
+        self.gid = try container.decode(UInt32.self, forKey: .gid)
+        self.umask = try container.decodeIfPresent(UInt32.self, forKey: .umask)
+        if let additionalGids = try container.decodeIfPresent([UInt32].self, forKey: .additionalGids) {
+            self.additionalGids = additionalGids
+        }
+        if let username = try container.decodeIfPresent(String.self, forKey: .username) {
+            self.username = username
+        }
+    }
 }
 
 public struct Root: Codable, Sendable {
     public var path: String
     public var readonly: Bool
 
+    public enum CodingKeys: String, CodingKey {
+        case path
+        case readonly
+    }
+
     public init(path: String, readonly: Bool) {
         self.path = path
         self.readonly = readonly
     }
+
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        self.path = try container.decode(String.self, forKey: .path)
+        self.readonly = try container.decodeIfPresent(Bool.self, forKey: .readonly) ?? false
+    }
 }
 
 public struct Mount: Codable, Sendable {
@@ -194,9 +299,18 @@ public struct Mount: Codable, Sendable {
     public var uidMappings: [LinuxIDMapping]
     public var gidMappings: [LinuxIDMapping]
 
+    public enum CodingKeys: String, CodingKey {
+        case type
+        case source
+        case destination
+        case options
+        case uidMappings
+        case gidMappings
+    }
+
     public init(
-        type: String,
-        source: String,
+        type: String = "",
+        source: String = "",
         destination: String,
         options: [String] = [],
         uidMappings: [LinuxIDMapping] = [],
@@ -209,6 +323,16 @@ public struct Mount: Codable, Sendable {
         self.uidMappings = uidMappings
         self.gidMappings = gidMappings
     }
+
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        self.type = try container.decodeIfPresent(String.self, forKey: .type) ?? ""
+        self.source = try container.decodeIfPresent(String.self, forKey: .source) ?? ""
+        self.destination = try container.decode(String.self, forKey: .destination)
+        self.options = try container.decodeIfPresent([String].self, forKey: .options) ?? []
+        self.uidMappings = try container.decodeIfPresent([LinuxIDMapping].self, forKey: .uidMappings) ?? []
+        self.gidMappings = try container.decodeIfPresent([LinuxIDMapping].self, forKey: .gidMappings) ?? []
+    }
 }
 
 public struct Hook: Codable, Sendable {