[DRAFT] Declare async state processing

[Zakelly]

this is only a draft, do not merge this

This is the draft of declaring processing interface introduced in FLIP-455

https://cwiki.apache.org/confluence/x/C4owEg

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CI report:

• 49444a3af468b40028f6737f98231c4b8c38c1e2 Azure: SUCCESS

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