Add a Gitlab plugin

Add a Gitlab plugin, this will allow to have some test that could interact with gitlab directly (when run in CI)

If CI env variable is not set all subsequent tests are skipped

I've created some fixture that seem interesting for me:

Checking a job status (if we have some tests dependent of the status of previous job) Downloading a given artifacts, this may be useful for test that will analyze some artifacts generated by sylvactl, debug on exit or else Retrieving logs from a job. Today I don't have real usecase for this but I keep it as an example of what we could do.

At some point it may be interesting to have some additional fixture to post a comment for example, to have report directly in MR for some very specific cases (slowness detection or else)