Add support for PaneInfo descriptor in Python SDK #18721
Description
The PaneInfo descriptor allows a user to determine which particular triggering emitted a value. This allows the user to differentiate between speculative (early), on-time (at end of window) and late value emissions coming out of a GroupByKey. We should add support for this feature in the Python SDK.
Imported from Jira BEAM-3759. Original Jira may contain additional context.
Reported by: ccy.