Scan execution policy: allow selection of branch type

What does this MR do and why?

This MR resolves Scan execution policy: allow selection of branc... (#414661 - closed) • Martin Čavoj • 16.2 • Needs attention (Deliverable)

Now scan execution policy can have either branch_type or specific branches in yaml structure

Screenshots or screen recordings

DESCRIPTION	VIDEO
New project	project_new
Existing project with validation	project_existing_validation
Existing group with validation	group_exsting_coliding_keys

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

How to set up and validate locally

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

1. Go to project or group
2. Secure -> Policies
3. Create new or edit existing scan execution policy
4. Branch type dropdown should have have new options and specific branches should add branches property instead of branch_type

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 #414657 (closed)