Added a flag to switch off GitLab API authentication

What does this MR do?

Make the GitLab Authentication configurable through the CLI flag. This is useful for local testing. The current implementation makes sure that the API workflow does not change. First, a token has to be issued by invoking the /authenticate endpoint -- the corresponding JWT token has to be passed on for subsequent request.

However, if GitLab auth is disabled, the provided credentials are no longer verified through the GitLab API.

What are the relevant issue numbers?

Add a flag to disable authentication (gitlab-org/gitlab#481549 - closed) • Julian Thome • 17.5 • On track

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests updated/added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer