Provide access to <K> within DoFn.OnTimerContext

org.apache.beam.sdk.transforms.DoFn.OnTimerContext Does not support access to Key. Current workaround:

@ProcessElement 

@StateId("dataKey") ValueState key

{ key.write(key)}

...

@OnTimer("processTimer") public void OnTimer(OnTimerContext otc,
@StateId("dataKey") ValueState key,

{key.read()}

Imported from Jira [BEAM-7155](https://issues.apache.org/jira/browse/BEAM-7155). Original Jira may contain additional context.
Reported by: rarokni@gmail.com.