Add a GitLab parser for git trailers

For &351 (closed) we need to parse Git trailer tags (https://git-scm.com/docs/git-interpret-trailers). This information is then used to determine if a commit should be included in the changelog, and to enrich the grouping of changes (e.g. by grouping all features together).

Our Markdown pipeline has some support for this in lib/banzai/filter/commit_trailers_filter.rb, but this is tied into our Markdown pipeline. Running a Markdown pipeline just to figure out what commits to include is too expensive, so we need direct/standalone access to a parser.

This parser should take as input a Git commit message, and produces a Hash of all Git trailers as output. The keys of this hash should be strings.