Remove unofficial skin sources and add skin event for extensions

[rtm516]

All unofficial cape and ears sources have been removed.

Ears/capes are now part of an extension GeyserMC/ThirdPartyCosmetics

New event SessionSkinApplyEvent for customising the skin, cape and geometry that's loaded for a player.

Needs documentation adding to the wiki and likely some testing to make sure I didn't break any skin stuff.

[Camotoy]

I'm not a big fan on doing a map lookup for connectionByUuid every time the event is fired, so if there's a way we can avoid that, that'd be stellar. Maybe we can just pass the player UUID?

[rtm516]

I'm not a big fan on doing a map lookup for connectionByUuid every time the event is fired, so if there's a way we can avoid that, that'd be stellar. Maybe we can just pass the player UUID?

We do pass the player UUID but likely some extensions will only want to modify java players and this is the only reliable way of checking, as other methods like checking version of the UUID only are reliable with floodgate, other auth types this falls apart on.

[onebeastchris]

Just noticed that a few annotations/no null checks are missing, those should probably be added :)