Expose instance level custom roles

What does this MR do and why?

This MR adds the possibility to list all custom roles using a GraphQL query. This is allowed only for instance administrators and is not possible on SaaS.

Database

Query plan

SELECT "member_roles".* FROM "member_roles" WHERE "member_roles"."namespace_id" IS NULL ORDER BY "member_roles"."name" ASC

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/24502/commands/78181

this query won't be running on gitlab.com as it is self-managed only feature.

How to set up and validate locally

1. Create few instance-level custom roles in the rails console (we have follow-up issues for API & UI)

MemberRole.create(name: 'Incident Manager', base_access_level: 10, read_vulnerability: true, admin_vulnerability: true)
MemberRole.create(name: 'MR admin', base_access_level: 10,  admin_merge_request: true)

# and one group-level custom role (possible also from group admin page)
MemberRole.create(name: 'Group admin', base_access_level: 10,  admin_group_member: true, namespace: Group.first)

2. Run the grapqhl query (https://gdk.test:3443/-/graphql-explorer) - not on SaaS - it should not be working on Saas

{
  memberRoles {
    nodes {
      id
      name
      readVulnerability
      readCode
      adminGroupMember
      enabledPermissions
    }
  }
}

Example response:

{
  "data": {
    "memberRoles": {
      "nodes": [
        {
          "id": "gid://gitlab/MemberRole/1",
          "name": "Custom",
          "readVulnerability": false,
          "readCode": false,
          "adminGroupMember": false,
          "enabledPermissions": [
            "ADMIN_MERGE_REQUEST"
          ]
        },
        {
          "id": "gid://gitlab/MemberRole/3",
          "name": "Custom",
          "readVulnerability": false,
          "readCode": false,
          "adminGroupMember": true,
          "enabledPermissions": [
            "ADMIN_GROUP_MEMBER",
            "ADMIN_MERGE_REQUEST"
          ]
        }
      ]
    }
  }
}

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 #417754 (closed)