Add support for branch exceptions in scan execution policy

What does this MR do and why?

This MR resolves #418743 (closed)

Description	MR
Add project branches dropdown	!127394 (merged)
Add branch exceptions for scan result policy	!127389 (merged)
Add branch exceptions for default rule	128312
Add branch exceptions for scan execution policy	👈 This MR

Screenshots or screen recordings

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

Description	Video
New project level scan execution policy	select exceptions project.mov
Scan execution policy project level validation	validation project.mov
Saved project level scan execution policy	project saved.mov
Group level no exceptions

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 scan execution policy or edit existing one
4. Select trigger or schedule in conditions section, make sure selected branch type and exceptions are selected
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 in 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)