Auth for multiple GitLab installations to the AI gateway.

This is coming from the discussion in gitlab-com/gl-infra/readiness!166 (comment 1448033317).

The GitLab instance is responsible for User authentication & authorization: Can this user in this group use AI-related features based on the License of the group. This is mainly applicable to GitLab SaaS, but self managed instances running the EE codebase also validate the installed license before reaching out to the AI gateway.

The AI-gateway itself also needs authentication: Can the GitLab instance use AI-features, based on the license of the GitLab instance making the request.

Ideally, the GitLab-instance would create a JWT-token based on a key that can be trusted and validated by the AI gateway