Add support for approval_settings and any_merge_request in policy schema

What does this MR do and why?

This MR adds support for approval_settings and any_merge_request rule type in the policy JSON schema.

How to set up and validate locally

1. Visit Secure -> Policies
2. Create a scan result policy with the following YAML:

   type: scan_result_policy
   name: Any MR
   description: ''
   enabled: true
   rules:
     - type: any_merge_request
       commits: unsigned
       branch_type: protected
   actions:
     - type: require_approval
       approvals_required: 1
       user_approvers_ids:
         -  1
   approval_settings:
     prevent_approval_by_author: true
     require_password_to_approve: true

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 #418752 (closed)