GraphQL API: Add `group` field to CiRunner
Related to #347856 (closed).
Background
In order to implement #347856[View_group_runner.png], we should provide the group of a given runner in our GraphQL API.
![#347856[View_group_runner.png]](https://gitlab.com/gitlab-org/gitlab/-/issues/347856/designs/View_group_runner.png){kind=link}
Description
Add a new nullable "group" field to the runner, which will be populated for group runners.
Edited by Miguel Rincon