Resolves#1672
This API is intended to expose useful heightmaps found in the server to API. This exposes all of the live world heightmaps currently in the server.
If a heightmap becomes impossible to implement, api spec allows the implementation to throw UnsupportedOperationException (far better than returning some weird unexpected value).
Tested via:
https://gist.github.com/Spottedleaf/5d47f67c55a9fb870251ff344bfeb6b3
