Runner controller management API (Part 3): Add endpoint to validate controller token (for GitLab Relay (previously KAS))

There should be an internal API that GitLab Relay (previously KAS) can access to verify AC's token. This API should return the ID of the AC (potentially more info in the future, like authn credentials that GitLab Relay should use). This is similar to the existing /api/v4/internal/agents/agent{k,w}/agent_info API endpoints and should accept JWT generated by GitLab Relay.