Register auth callbacks with XXMessengerClient

This MR allows registering auth callbacks with Messenger from XXMessengerClient library.

Callbacks can be registered at any time, but ideally should be registered before calling messenger.connect() to make sure all callbacks are handled. More than one AuthCallbacks can be registered, and all will receive callbacks, unless Cancellable object returned from registerAuthCallbacks is canceled (by calling cancel() on it, or when dealocated).

// first, register callbacks:
var cancellable: Cancellable? = messenger.registerAuthCallbacks(
  AuthCallbacks(handle: { callback in
    // implement auth callbacks handling
  })
)

// later, connect (which creates E2E and starts receiving callbacks):
messenger.connect()

// if registered callbacks are no longer needed:
cancellable?.cancel() // or: cancellable = nil

Sources/XXMessengerClient/Messenger/Functors/MessengerRegisterAuthCallbacks.swift

+import XXClient
+import XCTestDynamicOverlay
+
+public struct MessengerRegisterAuthCallbacks {
+  public var run: (AuthCallbacks) -> Cancellable
+
+  public func callAsFunction(_ authCallbacks: AuthCallbacks) -> Cancellable {
+    run(authCallbacks)
+  }
+}
+
+extension MessengerRegisterAuthCallbacks {
+  public static func live(_ env: MessengerEnvironment) -> MessengerRegisterAuthCallbacks {
+    MessengerRegisterAuthCallbacks { callback in
+      env.authCallbacks.register(callback)
+    }
+  }
+}
+
+extension MessengerRegisterAuthCallbacks {
+  public static let unimplemented = MessengerRegisterAuthCallbacks(
+    run: XCTUnimplemented("\(Self.self)")
+  )
+}