Add `role_approvers` enum value to policy schema

What does this MR do and why?

We want to Support Role Based Approval Action for Scan Result Policies.

This MR introduces the role_approvers enum property.

Example:

scan_result_policy:
- name: Require approvals for critical vulnerabilities
  description: ''
  enabled: true
  actions:
  - type: require_approval
    approvals_required: 1
    role_approvers:
      - developers
  rules:
  - type: scan_finding
    branches: []
    scanners: []
    vulnerabilities_allowed: 0
    severity_levels:
    - critical
    vulnerability_states:
    - detected

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #379057 (closed)