Improvements to struct interop for DOTS

[limoka]

This PR includes three improvements, which make working with DOTS based games much easier:

• Allow to inject custom structs into il2cpp runtime.
• A fix to how methods with struct pointers are generated.
• Allow Generic structs to be marked as blittable. This change could be breaking as some structs previously marked as classes now are structs. I'm performing checks if the struct is indeed blittable and making it's generic parameters constrained to unmanaged.

[limoka]

With the last commit, I think I fixed the generation issue Kasuromi found earlier. Now I added another kind of TypeSpecifics: GenericBlittableStruct, meaning blittability depends on used type arguments. For such structs I generate both boxed and unboxed versions, and switch them depending on the generic arguments.