This adds the `trailers` option to the project commits API. When
enabled, Gitaly will parse the Git trailers into a structured object,
which is then presented in the API response.

The API to obtain merge request commits doesn't support this option at
this time. Said API retrieves data from the database. The Gitaly RPC
used to populate that data doesn't support parsing Git trailers at this
time, and extending it will likely require a fair amount of work. As
such, we only extend the regular commits API for the time being.

See https://gitlab.com/gitlab-org/release-tools/-/merge_requests/1464
and https://gitlab.com/gitlab-org/release-tools/-/issues/500 for more
information.

Changelog: added