Split datagen into api and Create's actual datagen

[ChiefArug]

Currently datagen is a bit of a mix of stuff that should be API so that addons can utilise it and Create's actual datagen. This should be split up so it both easier for addons to use datagen and so it is clear how addons should use create's datagen.

This should be done by moving all of the currently abstract datagen classes to api (ie ProcessingRecipeGen, CreateRecipeProvider), and moving static methods from there like ProcessingRecipeGen#registerAll into another class. The current concrete implementations such as MillingRecipeGen should be split into an abstract class (with the useful helper methods) for addons to use (ie MillingRecipeGen) and a concrete class that create actually uses for its generation (ie CreateMillingRecipeGen and with the create specific helper methods). This should also be combined with making those concrete classes final as an extra nudge for addons to not use them.