Implement Code of Conduct handling in player configuration

[emmanuelvlad]

This PR introduces a new event-driven mechanism to handle the Code of Conduct (CoC) acceptance flow during the backend configuration stage for Minecraft 1.21.9+ clients.

The main changes add extensibility for sending and handling CoC packets, ensuring the proxy can pause configuration until the player accepts the CoC, and cleanly resume configuration afterward.

I am not entirely sure this is the best way to implement it. Let me know if there's a better way to do it, feedback appreciated!

---

Here is an implementation example on a proxy:

return func(e *proxy.PlayerConfigurationStartEvent) {
  payload, ok := getCodeOfConduct(e.Player().Settings().Locale().String()) // returns []byte, bool
  if !ok {
	return
  }
  e.SendCodeOfConduct(payload)
}