Draft: Add comparison tuning section for approval policies

What does this MR do and why?

Add a new section "Policy comparison tuning" for Approval policies with a checkbox to unblock approval rules by Scan execution policies.

Feature is controlled by feature flag unblock_rules_using_execution_policies.

References

Please include cross links to any resources that are relevant to this MR This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Backend MR: Add policy toggle unblock_rules_using_scan_exec... (!170605)
• Suggestion to add the toggle: #490092 (comment 2162119321)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

FF disabled	FF enabled	Popover

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:unblock_rules_using_execution_policies)

2. Visit a project -> Secure -> Policies
3. Create a new Approval policy
4. Verify that selecting the checkbox adds a policy_tuning section into the YAML and unselecting it disables the value

Related to #490092