Add support for branch exceptions in scan result policy

What does this MR do and why?

This MR resolves #418743 (closed)

This MR request adds possibility for a user to add branch_exceptions for scan result policy

It is now only available on project level

Description	MR
Add project branches dropdown	!127394 (merged)
Add branch exceptions for scan result policy	👈 This MR
Add branch exceptions for scan execution policy	TBD

Screenshots or screen recordings

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

Description	Recording
Saved policy editing
Group level - no exception option	group level.mov
New policy project level	new result policy.mov
Validation	validation.mov

How to set up and validate locally

Enable feature flag

Feature.enable(:security_policies_branch_exceptions)

1. Go to a project
2. Open Secure -> Policies
3. Create new policy or edit existing one
4. Select scan option in rules section
5. Go to a group
   1. Open Secure -> Policies
   2. Create new policy or edit existing one
   3. Make sure there is no exceptions option i UI

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.

Related to #418743 (closed)