Add a permanent identifier method to CommandSender

[SOF3]

Introduction

There is no reliable method to permanently identify a CommandSender. To permanently identify the console, the plugin has to check instanceof ConsoleCommandSender, but that assumes the knowledge that console is single-instance (or that other instances are supposed to imply the same thing as the console). To permanently identify a player, the plugin has to check for player UUID, which is not even in the CommandSender interface. To permanently identify plugin-defined CommandSender implementations... there is simply no reliable method.

Proposal

Introduce a getUniqueId method that returns ?UUID. If it returns null, the CommandSender cannot be uniquely identified. If it returns a UUID, users of CommandSender can rely on this value to identify a CommandSender over multiple sessions.

The ConsoleCommandSender unique ID can be generated from the server unique ID, which should be consistent throughout srever restarts.

Backward compatibility

This proposal is BC-breaking since CommandSender can be implemented