[Task]: Implement State in Go

[damccorm]

What needs to happen?

Part of #20510 - designed here - https://docs.google.com/document/d/1rcKa1Z6orDDFr1l8t6NA1eLl6zanQbYAEiAqk39NQUU/edit?usp=sharing

Issue Priority

Priority: 2

Issue Component

Component: sdk-go

[damccorm]

Generally, things that need to be done for this to be called complete:

Phase 1 - MVP

• User experience changes - make it possible to declare one or more state member variables. Perform validation. Plumb that information through to graph construction/execution. No/op, and temporarily throw a validation exception until more is implemented.
• Execution changes - when appropriate, pass in a fully hydrated state provider that knows how to handle ValueState (with set and clear operations) and can talk to the state api. Update validation to allow value state types.
• Integration test.

Phase 2 - Full support

Following pattern established in part 1, implement:

• BagState + integration test
• CombiningState + integration test
• SetState + integration test
• MapState + integration test
• Set and Map remove operations (per key)
• Clear operation
• Windowing specific integration test
• Documentation PR (at least drafted)
• CHANGES.md updated

Phase 3 - Cleanup

• Fix any hanging TODOs or create new issues for them