Expose runner managers in runner REST API

Users need the ability to check the runner managers associated with a runner, for instance to delete any that are offline. We could expose this information:

1. in API::Entities::Ci::RunnerDetails as an array of strings (we don't have many examples of arrays being returned in entities in our API); or
2. in a new GET /runners/:id/managers endpoint, which would return an array of the RunnerManager entity.

I believe 2 is the most idiomatic approach here, so we should go with that. We also already have an (internal) DELETE /runners/:id/managers/:system_id endpoint.