Description
Allow SplittableDoFns to self checkpoint.
Design doc: https://docs.google.com/document/d/1_JbzjY9JR07ZK5v7PcZevUfzHPsqwzfV7W6AouNpMPk/edit?usp=sharing
Feature is written E2E and users will be able to return ProcessContinuations from SDFs as of 2.39.0 but the full behavior has not been fully validated. An integration test that validates self-checkpointing is working as-intended will need to be written and passing before the feature is no longer considered experimental and this ticket is marked as resolved.