Add client callbacks for On Reconnect, On Shutdown and On Error

[lcsmuller]

Describe the feature

Add client callbacks for On Reconnect, On Shutdown and On Error. Clients should be able to deal with those events on-demand, as they might have a method of logging or resource management that requires listening to these events.

Acceptance criteria

• [ ] Add discord_set_on_xxx() methods for each one of the events

Version

v2.1.0