Group Chats

XXMessengerClient

• Add GroupRequestCallbacksRegistry and GroupChatProcessorRegistry
• Add MessengerStartGroupChat
• Update docs - describe how to start group chats

This MR does NOT contain functions for sending group messages, but it can be done directly on the group chat manager:

// register callbacks:
let groupRequestsCancellable = messenger.registerGroupRequestHandler(.init { group in
  // handle group request...
})
let groupChatProcessorCancellable = messenger.registerGroupChatProcessor(.init { result in
  switch result {
    case .success(let callback):
    // handle group chat processor callback...
    
    case .failure(let error):
    // handle error...
  }
})

// start group chat manager:
try messenger.startGroupChat()

if let groupChat = messenger.groupChat() {
  // Use XXClient.GroupChat API directly...
}