Add endpoint with standard roles and members count for self-managed

What does this MR do and why?

Id adds an endpoint with standard roles and members count for self-managed.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Database

New SQL query

Member.count_by_role

This query will not run on SaaS, only on self-managed. For Saas, we will filter by group or group hierarchy (follow-up MR will come, task: #472745 (closed)).

Query plans

• the query for the whole SaaS instance (it won't be running for SaaS)
• when filtered by group (gitlab.org)

How to set up and validate locally

Run the following graphQL query

{
  standardRoles {
    nodes {
      accessLevel
      name
      membersCount
    }
  }
}

Expected response

{
  "data": {
    "standardRoles": {
      "nodes": [
        {
          "accessLevel": 5,
          "name": "Minimal Access",
          "membersCount": 0
        },
        {
          "accessLevel": 10,
          "name": "Guest",
          "membersCount": 27
        },
        {
          "accessLevel": 20,
          "name": "Reporter",
          "membersCount": 41
        },
        {
          "accessLevel": 30,
          "name": "Developer",
          "membersCount": 34
        },
        {
          "accessLevel": 40,
          "name": "Maintainer",
          "membersCount": 40
        },
        {
          "accessLevel": 50,
          "name": "Owner",
          "membersCount": 85
        }
      ]
    }
  }
}

Related to #472745 (closed)