Ability to use non-player custom heads

[Poslovitch]

Description

Is your feature request related to a problem?

Originally asked by @BONNe in #477.

Using player heads is fun, but there are tons of heads out there that would really fancy up GUIs. Check out this website: https://minecraft-heads.com/custom-heads

Describe the solution you'd like us to implement.

All these "static heads" are stored on Mojang's servers but are not linked to any player account. We need some NMS reflection to fetch a "GameProfile" to apply to the head a skin out of a Base64 encoded String which provides a link to said skin.

There are already a few libs that do that. E.g. CS-CoreLib.

Describe alternatives you've considered.

Additional context

[BONNe]

And me :)
https://github.com/BentoBoxWorld/Challenges/blob/develop/src/main/java/world/bentobox/challenges/utils/HeadLib.java

[tastybento]

Maybe we can put in the lib that @BONNe used and then he'll be able to pull it out of Challenges?

Also, I hate to say it, but requesting any heads from Mojang's servers must be done carefully so as to not rate limit.

[BONNe]

My headlib is not perfect. It is just to get mob heads and nothing more. (only for guis)
But I would prefer to use proper BentoBox headLib, if someone provide it :)

[Poslovitch]

This will remain pending for now, until we have a clear plan of what to implement.