New change stream event: endOfTransaction.
In this ticket we only added support for it for unprepared (single-shard) transactions.
Full work on this event can be tracked via PM-2782

Description of Linked Ticket

When we see committed unprepared transaction in DocumentSourceChangeStreamUnwindTransaction, we can safely append no-op oplog entry about the end of this transaction to the stream.

This no-op entry should be filtered according to the namespaces, affected by the transaction.

This entry should be transformed into change stream event. 

Event should only appear if showExpandedEvents is true