One thing lead to another with this so this PR does a few related things.

• Exports the windowing strategy struct to configure aggregation stages in the element manager.
  • This is prep-work for fully supporting the rest of the windowing strategies + triggers.
• Uses the above to support AllowedLateness.
• In doing so, it was noted that incorrect conversions from integers (representing durations of nanosecond precision) to mtime.Time which is strictly a millisecond duration. This fix revealed two issues with Session Windows.
• Fixes to Session Window Merging
  • Merging wasn't correctly happening per key. Windows were being treated "globally".
  • We were incorrectly using the GapSize in prism. GapSize was already accounted for in the generation of the SessionWindow's interval Window representation.
  • These fixes resolved the issue that [prism] PrismRunnerTest::test_windowing - session windowing failing. #32085 was filed for, so the re-written Python test could be removed.
• Late data is now only dropped before aggregations, when it matters most.
  • This avoids a correctness issue dropping data in a Reshuffle.
  • This will be re-evaluated as windowing strategies are implemented, but it's better to have more tests enabled, so they can provide a signal.

Part of #31438

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