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/Utils/AuthCallbacksRegistry.swift

+import XXClient
+import XCTestDynamicOverlay
+import Foundation
+
+public struct AuthCallbacksRegistry {
+  public var register: (AuthCallbacks) -> Cancellable
+  public var registered: () -> AuthCallbacks
+}
+
+extension AuthCallbacksRegistry {
+  public static func live() -> AuthCallbacksRegistry {
+    class Registry {
+      var authCallbacks: [UUID: AuthCallbacks] = [:]
+    }
+    let registry = Registry()
+    return AuthCallbacksRegistry(
+      register: { authCallbacks in
+        let id = UUID()
+        registry.authCallbacks[id] = authCallbacks
+        return Cancellable { registry.authCallbacks[id] = nil }
+      },
+      registered: {
+        AuthCallbacks { callback in
+          registry.authCallbacks.values.forEach { $0.handle(callback) }
+        }
+      }
+    )
+  }
+}
+
+extension AuthCallbacksRegistry {
+  public static let unimplemented = AuthCallbacksRegistry(
+    register: XCTUnimplemented("\(Self.self).register", placeholder: Cancellable {}),
+    registered: XCTUnimplemented("\(Self.self).registered", placeholder: AuthCallbacks { _ in })
+  )
+}