[1.12.2] Terrain feature customisation functionality

[muzikbike]

Merger ticket of #40, #41, #45, #53, specifically for 1.12.2 as 1.17 should have support for all of these by default due to 1.16's changes but 1.12.2 may still receive legacy support.

Natural liquid springs #40
-defaults:
--overworld water and lava source blocks, as found in stone cliffs
--nether lava springs, as found coming from ceilings
--nether lava springs, hidden in netherrack
-any valid block ID
-ability to toggle between "in wall", "in ceiling" and "hidden" generation types

Disks #41
-defaults:
--clay (all biomes, 2-4, does not replace grass)
--sand (all biomes except swamps, 5-7)
--gravel (all biomes except swamps, 4-6)
-any valid block ID

Random patches #45
-very very many defaults, see #45 for some examples, alternatively look in the vanilla data pack files provided somewhere

Trees #53
-also many defaults