Repo aware monitors: add cm_last_searched table
Created by: camdencheek
This adds a table that will store the set of commits that were successfully searched. On subsequent runs, this set of commits (and all commits reachable from those commits) will be excluded from the search.
Currently unused, but will be exercised behind the cc-repo-aware-monitors
feature flag in subsequent PRs.
Stacked on #32518 Pulled from #32464
Test plan
Tests added for the new DB methods.