Allows customization in the Flink State binding

[xinyuiscool]

This patch allows LinkedIn to extend the state binding in FlinkStateInternals to support configs such as TTL.

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[alnzng]

LGTM. Looks like we will implement the customization in beam-runner seprately, right?

[xinyuiscool]

LGTM. Looks like we will implement the customization in beam-runner seprately, right?

Right, that's the plan. We will add our custom logic for ttl there.