added the api-calls for adding mergerequest dependencies to existing mergerequests

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Added the Api-Calls for fetching/adding/deleting MergeRequestDependencies to existing MergeRequests. The testdata is copied from the gitlab documentation at https://docs.gitlab.com/ee/api/merge_requests.html#get-merge-request-dependencies

The method CreateMergeRequestDependency currently doesn't parses the response object if any as the documentation gave a example response, but I always got an unknown error when adding the second parameter to "client.Do". So either the documentation is wrong or the interface io.Writer needs to be implemented correctly for the "MergeRequestDependency". But I didn't need the response so I wrote it without it for now. If anyone could give me a hint what is the correct way here, I will change it.