Support saving Python state with object-based checkpoints

Support saving Python state with object-based checkpoints

Allows SaveableObjects to specify feed dict addition callbacks for object-based saving.

For now just saves get_config() with Layers. Doesn't do any loading, and there isn't quite enough information to reconstruct a Model yet (needs topology).

My plan is to get Models to the point where they can be reconstructed from object-based checkpoints (probably one more change), add in SavedModel export (assuming no dynamic control flow for now), then add this "SavedModel+Python" format to Model.save / load_model.

PiperOrigin-RevId: 196043183