feat: add mtls support to gitlabstub

What does this MR do?

Add mutual tls support to gitlabstub.

Should unblock feat: support mutual TLS when calling the GitLa... (!663 - closed)

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines