Expose shared runner setting

What does this MR do and why?

This MR intends to address this issue: #340094 (closed)

When retrieving a group's details GET /groups/:id using the Groups API endpoint, we have an attribute for shared_runners_setting but it is currently not returned in the JSON response when doing a GET request. You can update the value to enabled/disabled using PUT /groups/:id with the attribute shared_runners_setting, but you're unable to return the current status.

In this MR, we expose the shared_runners_setting attribute and add relevant tests to check the value in multiple scenarios.

Changelog: changed MR: !124827 (merged)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

After:

The API response(Example: http://localhost:3000/api/v4/groups/33) includes shared_runners_setting value:

{"id":33,"web_url":"http://127.0.0.1:3000/groups/flightjs","name":"Flightjs","path":"flightjs","description":"Dolor tenetur nihil ipsum in quia nihil ut.","visibility":"public","share_with_group_lock":false,"require_two_factor_authentication":false,"two_factor_grace_period":48,"project_creation_level":"developer","auto_devops_enabled":null,"subgroup_creation_level":"maintainer","emails_disabled":null,"mentions_disabled":null,"lfs_enabled":true,"default_branch_protection":2,"avatar_url":null,"request_access_enabled":true,"full_name":"Flightjs","full_path":"flightjs","created_at":"2023-0605T12:42:13.678Z","parent_id":null,"shared_runners_setting":"disabled_and_overridable","ldap_cn":null,"ldap_access":null,"marked_for_deletion_on

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Checkout the feature branch and call the groups API endpoint: Example: http://localhost:3000/api/v4/groups/33
2. Notice that the response includes shared_runners_setting attribute status.

Kindly note that the shared_runners_setting attribute value changes based on the shared runners settings that are disabled/enabled for a group’s subgroups and projects.

Value	Description
enabled	Enables shared runners for all projects and subgroups in this group.
disabled_and_overridable	Disables shared runners for all projects and subgroups in this group, but allows subgroups to override this setting.
disabled_and_unoverridable	Disables shared runners for all projects and subgroups in this group, and prevents subgroups from overriding this setting.
disabled_with_override	(Deprecated. Use disabled_and_overridable) Disables shared runners for all projects and subgroups in this group, but allows subgroups to override this setting.

MR acceptance checklist

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