Add TypeHandlers to regeister default SaveArg/RestoreArgs.

[copybara-service[bot]]

Add TypeHandlers to regeister default SaveArg/RestoreArgs.

This allows users to save with Checkpointer.save(..., save_arg=None) (or restore with restore_arg=None), and have the argument automatically be converted to the default SaveArg/RestoreArg for each TypeHandler.