Address StateSpec inconsistency between Runner and Fn API

The ability to mix and match runners and SDKs is accomplished through two portability layers:
1. The Runner API provides an SDK-and-runner-independent definition of a Beam pipeline
2. The Fn API allows a runner to invoke SDK-specific user-defined functions

Apache Beam pipelines support executing stateful DoFns[1]. To support this execution the Runner API defines multiple user state specifications:

• ReadModifyWriteStateSpec
• BagStateSpec
• OrderedListStateSpec
• CombiningStateSpec
• MapStateSpec
• SetStateSpec

The Fn API[2] defines APIs[3] to get, append and clear user state currently supporting a BagUserState and MultimapUserState protocol.

Since there is no clear mapping between the Runner API and Fn API state specifications, there is no way for a runner to know that it supports a given API necessary to support the execution of the pipeline. The Runner will also have to manage additional runtime metadata associated with which protocol was used for a type of state so that it can successfully manage the state’s lifetime once it can be garbage collected.

Please see the doc[4] for further details and a proposal on how to address this shortcoming.

1: https://beam.apache.org/blog/stateful-processing/
2: https://github.com/apache/beam/blob/3ad05523f4cdf5122fc319276fcb461f768af39d/model/fn-execution/src/main/proto/beam_fn_api.proto#L742
3: https://s.apache.org/beam-fn-state-api-and-bundle-processing
4: http://doc/1ELKTuRTV3C5jt_YoBBwPdsPa5eoXCCOSKQ3GPzZrK7Q