Missing support for multiple runners with same ID/token in REST API v4

Since 16.2 (or something like that) multiple runners can share the same runner token and ID (via the new registration flow). However, this is not reflected in the /runners resources in REST API v4.

When I start two gitlab-runner instances on two different machines with the same token, I can see in the web UI that the runner has two runner instances, each with its own system ID and IP address. But the /runners endpoint doesn’t provide this information, there’s no system_id and only one ip_address.

Related issues: #430809