Add "custom_model_data" to blocks

As of 1.21, we have the ability to give items and entities Models without changing all models for that item/entity with Custom Model Data. However, The most needed application of Custom Models is for blocks, as we have no performance friendly way to add custom blocks in vanilla Minecraft.

We have the ability to give the illusion of custom blocks with interaction entities or armor stands. However these require an already existing block to have a hitbox, which is janky. The only other alternative is to use invisible shulkers, but these don't work with non full blocks such as stairs or slabs, as the shulker head doesn't turn invisible. All of these methods have setbacks and are very unfriendly to performance in large quantities.