Initial implementation of GitLab provider

[JAORMX]

Summary

This does a very minimal implementation of the gitlab provider.

It merely handles policy and enrollment. Which is progress. Note there is more to do to get this ready, but this is just the start. For safety, the gitlab provider is set behind a feature flag, so it cannot be enrolled.

Piggy backing on this, I also set the dockerhub provider to be behind a feature flag.

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 53.844% (-0.6%) from 54.402%
when pulling 5e14224 on gitlab
into 082572d on main.

[dmjb]

General thought (not a blocker for this PR) - this rttst.go needs to get changed every time we touch providers, the executor, and various other things. I wonder if there's some way we can structure this to avoid needing to change it so often. (Obviously the big challenge is that needs to work without the database so it can't reuse a lot of the abstractions we have in the server)

[JAORMX]

Yeah, that's the tricky side. Gotta give it some thought as we add more providers.

[eleftherias]

Looks great!