Add runner registration token in groups api response

Problem to solve

Add the runners_token in the response for details of group API

Intended users

Developers, DevOps Engineer, System Administrators

Further details

We provide the same information in the project API which is inconsistent. This will help increase automation for registering tokens, instead of hardcoding the registration token users can get it via the API before registering the runner.

Permissions and Security

Anyone who has maintainer access and can view the settings page inside of the UI

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/issues/24030#note_162428910