Mark SandboxSnapshot init as public for sandbox plugins (#309)

Changed `SandboxSnapshot` initializer from `package init()` to `public
init()` to enable external package consumers to create and use
`SandboxSnapshot` instances in their custom Sandbox plugins.

The `SandboxSnapshot` is used for `container list` functionality

Sources/ContainerClient/SandboxSnapshot.swift

@@ -25,7 +25,7 @@ public struct SandboxSnapshot: Codable, Sendable {
     /// Containers placed in the sandbox.
     public let containers: [ContainerSnapshot]
 
-    package init(
+    public init(
         status: RuntimeStatus,
         networks: [Attachment],
         containers: [ContainerSnapshot]