Remove CodegenDataModelInstance and make applications provide their codegen data model (likely via a static for example)

[andy31415]

See #36658 (comment)

The usage of a mutable persistence provider that is used at startup is awkward. It seems better to have the persistence provider established in the constructor.

Expected changes:

• Make the persistence provider to use part of the CodegenDataModel constructor (may be null, which means unchanged)
• Remove CodegenDataModelProviderInstance (header and cpp files and all linkages), let applications build their own codegen data model provider instead and allocate space for them.

[andy31415]

Note that this means a LOT of file changes, however making applications be responsible of what data model provider they use seems to be the best dependency use (this way it is clear that applications use zap/ember for example, so they provide that one as their underlying model)

[bzbarsky-apple]

This is also the sanest way to have applications have a provider that uses ZAP codegen for some endpoints and dynamic things for others.