* feat: Support Checkpoint Serialization
* Implements serialization roundtripping for checkpoints.
* Adds support for JSON serialization
* Adds FileSystem-based checkpoint persistence
* fix: Executor State does not deserialize correctly
The StateManager was not properly handling delay-deserialized values.
* Fix PortableValue handling in StateManager (this makes it delegate to PortableValue the uwnrapping)
* Fix UnitTest to actually test checkpoint serialization
* Additional review comment fixes
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Co-authored-by: Chris <66376200+crickman@users.noreply.github.com>
Jacob Alber