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

Docs/XXMessengerClient.md

@@ -27,6 +27,9 @@ let messenger: Messenger = .live(environment)
 Example:
 
 ```swift
+// allow cancellation of auth callbacks registration:
+var authCallbacksCancellable: Cancellable?
+
 func start(messenger: Messenger) throws {
   // check if messenger is loaded:
   if messenger.isLoaded() == false {
@@ -42,6 +45,13 @@ func start(messenger: Messenger) throws {
   // start messenger's network follower:
   try messenger.start()
 
+  // register auth callbacks before connecting:
+  authCallbacksCancellable = messenger.registerAuthCallbacks(
+    AuthCallbacks(handle: { callback in
+      // implement auth callbacks handling
+    })
+  )
+
   // check if messenger is connected:
   if messenger.isConnected() == false {
     // start end-to-end connection: