Add custom_roles to security policies API

What does this MR do and why?

This MR updates the existing graphql and REST APIs with custom_roles as we will be supporting custom roles as approvers with &13550 (closed)

References

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• #457796 (closed)

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

• Create a group (make sure that there are members in the group) and a project within it
• Create custom role for the instance (GDK is self-hosted) following the docs
• Create a security policy (Merge Request approval policy) for the group by going to Secure->Policies and toggle to the YAML mode and update the policy content:

name: Custom Role
description: ''
enabled: true
actions:
- type: require_approval
  approvals_required: 1
  role_approvers:
  - developer
  - 1
- type: send_bot_message
  enabled: true
rules:
- type: any_merge_request
  branch_type: protected
  commits: any
approval_settings:
  block_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

• Go to http://gdk.test:3000/-/graphql-explorer and execute the query:

{
  project(fullPath:<project path>) {
    approvalPolicies {
      nodes{
        actionApprovers {
          customRoles{
            id
            name
          }
        }
      }
    }
  }
}

Related to #505167 (closed)