Improve interplay between PushbackSideInputRunner and GroupAlsoByWindowViaWindowSetDoFn

This originated from a discussion on a PR: https://github.com/apache/beam/pull/2235

`GroupAlsoByWindowViaWindowSetDoFn`/`GroupAlsoByWindowViaWindowSetNewDoFn` and `PushbackSideInputDoFnRunner` don't work well together and we manually need to explode windows in `FlinkStreamingTransformTranslators.ToKeyedWorkItem` because of this:

- `GroupAlsoByWindowViaWindowSetDoFn` is a `DoFn, KV>` so you have to push in `KeyedWorkItem`. These themselves contain `WindowedValue` (or timers).
- For executing a `DoFn` we use a `DoFnRunner`. For our problem the interesting case is using a `PushbackSideInputDoFnRunner`. The interesting method is `processElementInReadyWindows(WindowedValue elem)` where `InputT` is the input type of the `DoFn` which, for the windowing case, is `KeyedWorkItem` (from above). The actual expanded type signature is thus `processElementInReadyWindows(WindowedValue> elem)` where the keyed work items again contain `WindowedValues` (again, from above).
I think the `PushbackSideInputDoFnRunner` was not initially meant for executing `GroupAlsoByWindowViaWindowSetDoFns`.

Imported from Jira [BEAM-1850](https://issues.apache.org/jira/browse/BEAM-1850). Original Jira may contain additional context.
Reported by: aljoscha.

Start with PushbackSideInputDoFnRunner.processElementInReadyWindows, GroupAlsoByWindowViaWindowSetDoFn and GroupAlsoByWindowViaWindowSetNewDoFn, then inspect FlinkStreamingTransformTranslators.ToKeyedWorkItem. Read the discussion in PR 2235 and the imported BEAM-1850 context to determine the intended interaction. Done should remove the need to manually explode windows for this combination.

Written by the indexing model from the issue text.