Bindings for stateful DoFn

[nevillelyh]

POC here: https://github.com/spotify/scio/tree/neville/state

It's possible but with the current approach we can only have finite hard coded number of states a single DoFn can have. Alternatively we might have to code-gen DoFn with @State annotations but not sure how feasible is that or if it'll play well with Beam's annotation processor.

We should also save this for 0.10.0 after the (K, V) coder revamp (#3056) if we decide to do so.