Add Tickets that were acknowledged to the response from AcknowledgeBackfill

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Is your feature request related to a problem? Please describe.

When acknowledging a backfill to assign tickets to it, the assigner (the caller of AcknowledgeBackfill) never gets a list of tickets that were successfully acknowledged.

Describe the solution you'd like

A list of tickets associated with the backfill to be returned from AcknowledgeBackfill.

Describe alternatives you've considered

A hacky self-made tracking method that can break in many ways.

Additional context

Not getting a list of tickets that were successfully assigned breaks with the model of how Assigning works because the normal Assignment process has an explicit list of failures and an implicit list of successes (each requested ticket that was not on the failure list). In Backfill Ack Assignment, there is an indefinite (but bounded) amount of time between the FetchMatches call completing and the AcknowledgeBackfill call starting. This means that tickets could be cancelled in that time, reducing the set from what FetchMatches returned. It is also valuable to be able to acquire the ticket data from the acknowledging party to consume data that was stored in the Extensions of the tickets.