R1:b6fa009cf00a

Enrich "priority" transactions in Maniphest for "transaction.search"

Summary:
Ref T13187. See <https://discourse.phabricator-community.org/t/task-priority-change-info-missing-in-firehose-webhook/1832/2>. We can reasonably enrich these transactions.

Since priorities don't have unique authorative string identifiers, I've mostly mimicked the `maniphest.search` structure.

Test Plan: Called `transaction.search` on tasks which were: created normally, created with a priority change, saw a priority change after…