Add `branch_exceptions` to security policy schema

What does this MR do and why?

Adds support for branch_exceptions to the security policy schema (see &9567 (closed)).

How to set up and validate locally

• On the project or group level, navigate to Secure > Policies and verify you can create the following two sample policies:

  type: scan_execution_policy
  name: 'Test'
  description: ''
  enabled: true
  rules:
    - type: pipeline
      branch_type: protected
      branch_exceptions:
        - master
        - { name: "develop", full_path: "foo/bar" }
  actions:
    - scan: container_scanning
      tags: []

  type: scan_result_policy
  name: Test
  description: ''
  enabled: true
  rules:
    - type: scan_finding
      scanners:
        - sast
      vulnerabilities_allowed: 0
      severity_levels: []
      vulnerability_states: []
      branch_type: protected
      branch_exceptions:
        - master
        - { name: "develop", full_path: "foo/bar" }
  actions:
    - type: require_approval
      approvals_required: 1
      user_approvers_ids:
        - 11

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 #418735 (closed)