Enable de/serialization of nested control flow.

This is a follow-up to the previous commit
(https://github.com/tensorflow/tensorflow/commit/23851760b7b099214bdd4f1b88156d7ac2bdd2a2).
It adds the new proto schemas, enables the behavior for reading and
writing the new protos, and adds a test for de/serializing nested
while loops.

There's still a bug preventing deserializing conds, which will be addressed
in another change.

PiperOrigin-RevId: 187082713