Expose a top level user agent PipelineOption which can be communicated to external services

This concept is used by Bigtable and Dataflow service to specify what version of the SDK is being used and is currently available through ReleaseInfo as a static property.

The Dataflow distribution attempts to override this but is unable to propagate this user agent to dependent modules cleanly. Having dependent modules get the user agent from a PipelineOption would make it possible for a runner to modify the user agent during execution to be able to identify its flavor.

It seems likely that Flink/Spark/... would like to modify the user agent as well for the same purpose.