This adds tracking of merged requests when creating deployments. These
changes are hidden behind a feature flag at the moment, and no UI or
APIs have been added yet.

By tracking merge requests, developers will eventually be able to easily
see when their merge requests made it into a certain environment. This
data also allows for more accurate measuring of the time between a merge
request getting merged and deployed to a certain environment.

See https://gitlab.com/gitlab-org/gitlab/issues/32584

 for more
information.

Co-authored-by: Alessio Caiazza <acaiazza@gitlab.com>