Refactor common portable runner infrastructure for better reuse

The Flink runner is currently Beam's most mature portable OSS runner. Much of the Flink portable runner's implementation details are not unique to Flink, and yet are confined to the Flink runner code. In order to ease development on other portable runners such as the Spark runner, this reusable code should be moved into some common location.

I've set this up to track my progress on these ongoing improvements.