Authenticate `Runtime::Feature` as admin

What does this MR do?

Uses an admin access token if one is provided (via GITLAB_QA_ADMIN_ACCESS_TOKEN) when enabling/disabling feature flags.

Otherwise it uses the admin username and password via Runtime::Env. Those credentials fallback on the default username and password if GITLAB_ADMIN_USERNAME and GITLAB_ADMIN_PASSWORD are not set, so it checks if the user with those credentials has admin access, and raises an exception if it doesn't.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content