Add approver for group level scan result policies

What does this MR do and why?

Add approver for group level scan result policies:

1. Extends FetchPolicyApproversService to support users from the scope of a group.
2. Refactor FetchPolicyApproversService calls to be aligned with the previous change.
3. Use FetchPolicyApproversService to fetch approvers from the policy controller (at the group level).

This change is going to be used by the frontend code as part of this MR.

Related issue: #367716 (closed)

How to set up and validate locally

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

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