Implement a notchian skin server

[alikindsys]

This would allow for player skins to persist across different servers.

We would have to take into consideration existing skins for users who bought the game, where we delegate to Mojang's API until said user sets a skin ou our skin server (and gets marked as dirty and preferred to Mojang's skin server).

It might be nice for it to be version agnostic, and implement a way to "steve-ify" modern thin skins, so even moving to an older server (e.g. GregTech: New Horizons) players' skin would look mostly normal.

[alikindsys]

This would be implemented in a way that only changing the skin server on the server settings would do the trick, requiring little to no changes to the mod as possible.

A separate mixin-only mod might be needed for integrating any existing player skin mods with the new API, as we wouldn't implement an session server (yet).