Add some infrastructure for type registrations, use it to revive dictionaries

Lots of boilerplate for handling version numbers. This should make it reasonably easy to maintain backwards compatibility by appending new wrapper versions over time.

The main reason I want this is so we can eventually save a .signatures attribute without special casing, which requires dictionaries be revived as such. But hopefully the same infrastructure will work for Keras Layers. Those will need their own proto format, presumably another oneof in SavedUserObject.

PiperOrigin-RevId: 228919938