Block and Items custom 3D models

Verification builds:
Build 332621 (and all previous)

Summary:

> This is not a bug but a feature request, mandatory to allow content creators do advanced things (Addons creators as well as TexturePack creators)

In Minecraft PC, we can customize the 3D model of any block and any item with a JSON format.
Block models also have a blockstate format, allowing to setup what 3D model to use for each block, its rotation, random rotations, random 3D models...

Impact:

For PureBDcraft as example, we can't customize MCPE as in MCPC, including the swords, fence gates, bookshelves, cauldron, pumpkin/melon stem, randomize dirt/grass/cobblestone/wood planks, redstone wires/repeaters/lamp, trapdoors, noteblock, ...

Observed Results:

Content creators can't customize block models or item models.

Expected Results:

Be able to customize 3D models for blocks and items with exact same format as MCPC (blockstate json + 3D model json).

Screenshots/Videos attached: No