Ensure two containers cannot use the same DNS hostname. (#490)

- Closes #150, #394.
- Introduces `AttachmentConfiguration` type so that we can add key-value
options to `--network` in the future.
- Eliminates redundant `ContainersService.Item` type.
- Since we now ensure at ContainersService that hostnames will not
conflict, the network helper IP allocator now simply provides the
existing IP if for an allocation on an existing hostname, which should
handle (in an eventually consistent way) the case where a container
fails to deallocate an IP on shutdown.

Sources/ContainerPersistence/DefaultsStore.swift

@@ -28,6 +28,7 @@ public enum DefaultsStore {
         case defaultInitImage = "image.init"
         case defaultKernelURL = "kernel.url"
         case defaultKernelBinaryPath = "kernel.binaryPath"
+        case defaultSubnet = "network.subnet"
         case buildRosetta = "build.rosetta"
     }
 
@@ -85,6 +86,8 @@ extension DefaultsStore.Keys {
                 return "vminit:latest"
             }
             return "ghcr.io/apple/containerization/vminit:\(tag)"
+        case .defaultSubnet:
+            return "192.168.64.1/24"
         case .buildRosetta:
             // This is a boolean key, not used with the string get() method
             return "true"