GraphQL: Add new runner field for with token that is used in registration

Descripiton

After creation at #387808 (closed), runners have no runner machines assigned to them.

In order for the frontend to present a glrt- token so the user can start the registration, we should provide them with a token:

• This token can only be seen by the user that created the runner (creator).
• Only glrt- prefixed tokens are made available here;
• This token can only be seen for a period of 3 hours after the creation of the runner;
• (optional) The token becomes unavailable once the user has registered a machine to it

query getRunner($id: CiRunnerID!) {
  runner(id: $id) {
    ephemeralAuthenticationToken # new! 
  }
}