Merge submodule API into the commit API
For updating submodules references we have a different endpoint at the moment (https://docs.gitlab.com/ee/api/repository_submodules.html#update-existing-submodule-reference-in-repository).
Nevertheless, there are other related issues like https://gitlab.com/gitlab-org/gitlab-ce/issues/46507 where we may need the update to be included into a multi action commit using the commit API.
We should consider merging the submodules API endpoint into the commits endpoint.