Add branch_type validations

What does this MR do and why?

This MR adds validation for the new branch_type that we're adding in &9468 (closed).

The validation is only applied when the feature flag security_policies_branch_type is enabled.

Screenshots or screen recordings

How to set up and validate locally

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

1. In rails console enable the feature flag

   Feature.enable(:security_policies_branch_type)

2. Go to Settings -> Repository -> Protected branches and unprotect the main branch
3. Add a new scan execution policy with the following yaml:

   type: scan_execution_policy
   name: Pipeline / Protected Branches / Secret Detection
   description: ''
   enabled: true
   rules:
     - type: pipeline
       branch_type: protected
   actions:
     - scan: secret_detection

4. Clicking on Configure with a merge request should give an error Branch types don't match any existing branches.

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 #404777 (closed)