Fix Create/Update Mutation Resolvers

Given a one-to-many or many-to-many relationship between types A and B, the generated create/update mutation of type A takes an id or list of ids as references to instances of B. However, the generated resolver doesn't account that properly.

Update the codegen for these resolvers such that the instances of B are obtained by their primary keys and then appended to respective collection of A model.

Besides that, for types with complex primary keys the respective input type may be generated, so that referencing of another type is more clear and convenient. However, it may produce a lot of duplicating input types if generated per type basis. But reusing an input type may be problematic as well in the way how to name the input type.