Support withCredentials on GitLabApiToken(Impl)

[tiago-teixeira5]

Fixes #536

Inspired #878 , turns out it was missing the MultiBinding implementation for the GitLabApiTokenImpl class GitLabApiToken interface.

During tests (mvn verify), there was an unexpected failure on GitLabConnectionConfigTest due to my machine having port 80 being listened to. Changing the port to an uncommon port (47 is reserved but apparently not assigned) allowed to circumvent that.

Testing done

A test was added that runs the full pipeline for testing withCredentials, binding with both the full $class and the gitlabApiToken name.

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[krisstern]

Thanks @tiago-teixeira5! I will give this a review over the next few days