Start on a 1.x-compat codepath for tf.saved_model.load

Allows selecting between multiple MetaGraphs and loading them into 2.x. Variables are OK as long as they're resource variables. Signatures are available in the same format as for 2.x-exported SavedModels, concrete functions which take only keyword arguments and return dictionaries.

Stuff that still needs doing:
  - Lift any variables at all out into the eager context
  - Fix ref variables (running the restore function isn't initializing them for some reason)
  - Assets

PiperOrigin-RevId: 233135086