Add gitlab-cloud-connector library dependency

What does this MR do and why?

This MR is step 3 in extracting a new JsonWebToken type that is specific to Cloud Connector:

• Step 1: Introduce the new JWT implementation in gitlab-rails and roll it out behind a FF: !172378 (merged) ✅
• Step 2: Introduce this implementation in the new gitlab-cloud-connector Ruby gem: gitlab-org/cloud-connector/gitlab-cloud-connector!44 (merged) ✅
• Step 3: This MR: Use this implementation from the library, and remove the copy from this codebase.

In a last step, I will then also remove the FF, but I think it makes sense to keep it for now; in case something goes wrong with this, we can still fall back to the original implementation.

References

Please include cross links to any resources that are relevant to this MR This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• gitlab-org/cloud-connector/gitlab-cloud-connector#30 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

There should be no visible change in behavior; it's just moving code around.

A good check to perform is to make sure all AI features still work as intended. To test this, follow https://docs.gitlab.com/ee/development/ai_features/ and then:

1. Set CLOUD_CONNECTOR_SELF_SIGN_TOKENS=1 and boot Puma
2. Start the AI gateway locally and make sure rails is set up to talk to this instance (see docs)
3. Make sure you have at least a Premium license and have a Duo add-on bought.
4. Go to Settings > Admin > Gitlab Duo and let the Health check run. It should be green.