In APIv1 we were using UpdateJob to verify if the runner exists. It was
the only method that was using Runner's token and used in special way
had no side effects (like scheduling a new job or unregisterring a
Runner).

In APIv4 we've change UpdateJob to use job's token as authentication
credentials, and that way we've removed the only endpoint that could
be used to verify if the Runner with a certain token exists in target
GitLab installation.

This commit adds `POST /api/v4/runners/verify` endpoint whose only
responsibility is to respond if Runner with posted credentials exists or
not.