Move Keras to SavedModel code and some SavedModel utils into core.

Overall:
- Moved functions and constants from various parts of the Estimator codebase
- Exposed the save/load code as tf.keras.experimental.export and tf.keras.experimental.load_from_saved_model

In upcoming changes:
- Move code from keras/engine/saving.py into the keras/saving folder
- Resolve circular dependency between SavedModels and Keras, so saving can be added into the engine target.
- Remove copied fuctions from tensorflow_estimator

PiperOrigin-RevId: 225624440