Adds some new methods to GatewayHandle:

• identify, which functions like send_identify, but waits to receive the response
• resume, which functions like send_resume, but waits to receive the response
• request_guild_members, which functions like send_request_guild_members, but waits to receive all GuildMembersChunks
• update_voice_state, which functions like send_update_voice_state, but waits up to 1 second to receive a potential response
• request_last_messages, which functions like send_request_last_messages, but waits to receive the response

These work by creating a temporary observer, which sends the event through either a tokio::sync::broadcast or tokio::sync::oneshot channel. These two observer types are now also part of the public api as BroadcastEventObserver<T> and OneshotEventObserver<T>

It also adds Events.session.resumed and GatewayResumed for the Resumed dispatch event. I'm still not sure if resuming is practically possible, since you need to provide the last received sequence number