[Task]: Add MultimapState support for Beam and Dataflow

[zhengbuqian]

What needs to happen?

Define multimap state API and add the implementation for InMemory and Dataflow/Windmill.

Issue Priority

Priority: 2

Issue Component

Component: runner-dataflow

[zhengbuqian]

.take-issue

[lukecwik]

I see that you have started on an implementation but have you shared your intentions and a design for the API with dev@beam.apache.org?

[zhengbuqian]

Hi Lukasz, I didn't but I have just sent a brief doc of MultimapState's interface to dev@beam.apache.org. Thanks for reminding!