Add custom_role_approvers to MR approval policies

What does this MR do and why?

This MR adds a migration to add custom_roles to scan_result_policies table and updates the service classes to persist it from the security policy.

References

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• #457796 (closed)

MR acceptance checklist

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Before	After

How to set up and validate locally

• Create a custom role for the instance following the docs and note the ID of the custom role
• Create a MR approval policy with the ID of custom role in role_approvers

type: approval_policy
name: Custom Role Approver Policy
description: ''
enabled: true
policy_scope:
  projects:
    excluding: []
rules:
  - type: scan_finding
    scanners: []
    vulnerabilities_allowed: 0
    severity_levels: []
    vulnerability_states: []
    branch_type: protected
actions:
  - type: require_approval
    approvals_required: 1
    role_approvers:
      - developer
      - 5
  - type: send_bot_message
    enabled: true
approval_settings:
  block_branch_modification: true
  block_group_branch_modification: true
  prevent_pushing_and_force_pushing: true
  prevent_approval_by_author: true
  prevent_approval_by_commit_author: true
  remove_approvals_with_new_commit: true
  require_password_to_approve: false
fallback_behavior:
  fail: closed

• After the policy is created verify that the Security::ScanResultPolicyRead has custom_roles value populated with the ID:

Security::ScanResultPolicyRead.last.custom_roles

Addresses #457796 (closed)