Configuration to add custom git commit trailers

[esamson]

Right now we use commits.message to have commits generated by scala-steward play well with other git integrations we have in our CI pipeline. This mostly works except for commits generated by post-update hooks since we cannot control the commit message there.

It would be good to be able to configure the commit message for an update hook, like the scalafmt one for example. But maybe a more general approach is to have a configuration for a list of trailers that will be added to all git commit messages -- both the regular one and those from extra hooks.

I don't really mind the existing messages but I would like to be able to add specific content to be able to drive my other git integrations.

[fthomas]

I don't mind having repo configuration for adding commit trailers to CommitMsg.