Allow the creation of scan result policies

What does this MR do and why?

Initial steps towards allowing the creation of scan result policies for group level including feature flag. Only up to MR creation.

Related issue: #367713 (closed)

Screenshots or screen recordings

Policy selection

Branch selection on group level

Approvers selection on group level

How to set up and validate locally

1. Enable group_level_scan_result_policies feature flag
2. Go to the group to be tested and select Security & Compliance -> Policies
3. Click on New Policy and select Scan Result Policy
4. After filling up the required details select Configure with a merge request

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.