WindowingStrategy, Window fields in the project as a trait of a Rel

Currently, WindowingStrategy is just a property of a PCollection but is unknown during planning. As with boundedness / unboundedness, this should be available statically so it can affect the plan. It may not be literally a WindowingStrategy object, but a SQL-level equivalent.

One windowing-related thing that is interesting is which fields represent the window. If we know this, then we know we can support per-window equijoins as long as a join condition also does an equijoin on the window.

Imported from Jira [BEAM-4721](https://issues.apache.org/jira/browse/BEAM-4721). Original Jira may contain additional context.
Reported by: kenn.