Optionally include trailers in the commits API

What does this MR do?

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 release-tools!1464 (merged) and https://gitlab.com/gitlab-org/release-tools/-/issues/500 for more information.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.