Add read_runners custom ability

What does this MR do and why?

This MR allows accessing the runner list and runner fleet dashboard by adding a read_runners custom ability to custom member roles.

This is useful for the Engineering Productivity team as a few of the team members only have Developer baseline permissions (plus some custom permissions) in the gitlab-org group. (See discussion (internal).)

Somewhat related to #424171.

Closes #468202 (closed)

Screenshots

How to set up and validate locally

1. Create a member role with the Read runners custom permission
2. Go to a group
3. Create a runner (that’s online!)
4. Add a member with the custom member role
5. Impersonate the user
6. Go to Build > Runners in the group
7. See that the list is visible

For the fleet dashboard, follow the steps above and:

1. In the Rails console, run (with group_id to your ID):

   Feature.enable(:runners_dashboard_for_groups, Group.find(group_id))

2. Go to Build > Runners in the group
3. Click on Fleet dashboard