Key encoding for state requests is not consistent across SDKs

The Flink runner requires the internal key to be encoded without a length prefix (OUTER context). The user state request handler exposes a serialized version of the key to the Runner. This key is encoded with the NESTED context which may add a length prefix. We need to convert it to OUTER context to match the Flink runner's key encoding.

So far this has not caused the Flink Runner to behave incorrectly. However, with the upcoming support for Flink 1.9, the state backend will not accept requests for keys not part of any key group/partition of the operator. This is very likely to happen with the encoding not being consistent.

*NOTE* This is only applicable to the Java SDK, as the Python SDK uses OUTER encoding for the key in state requests.