API: Rename to NetworkClient, client as instance. (#1405)

- Part of #1404.
- COMPATIBILITY: Breaking change to client API. XPC protocol and
persistent data remain fully compatibile.
- Renames `ClientNetwork` to `NetworkClient`, aligning with
`ContainerClient`.
- Client uses instance methods instead of utility functions, with an
`init(serviceIdentifier:)` for the XPC service name. The value is
currently unused.

Sources/ContainerCommands/Builder/BuilderStart.swift

@@ -265,7 +265,8 @@ extension Application {
             // Enable Rosetta only if the user didn't ask to disable it
             config.rosetta = useRosetta
 
-            guard let defaultNetwork = try await ClientNetwork.builtin else {
+            let networkClient = NetworkClient()
+            guard let defaultNetwork = try await networkClient.builtin else {
                 throw ContainerizationError(.invalidState, message: "default network is not present")
             }
             guard case .running(_, _) = defaultNetwork else {