Make submodules API more powerful

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There should be an API which allows listing the submodules (remote url, git ref) for a specific git ref of a Gitlab project. There are projects working around this python-gitlab-submodules but they require about 4 API calls per submodule which is quite resource intense. Therefor, it would be nice if this feature could be provided by the Gitlab API directly. The current submodules API only supports updating the git ref of an existing submodule.