GraphQL API: Add "runners" to groups

Description

This issue is similar to #328808.

In #328518 (closed) we defined a list of runners for administrators, and now we should do the same for groups and projects.

Now, we should add a new API to:

2. Fetch runners available to a group (or namespace)

  # Return all runners that are direct members of this group (`membership: "DIRECT"`) or of desdendant groups/projects (`membership: "DESCENDANTS"`)
  # -  Users should be owners of this group.
  group(fullPath: "my-group" ) {
    runners (membership: DESCENDANTS) {
      nodes {
        id
      }
    }
  }