Expose single member role in GraphQL

What does this MR do and why?

It adds a graphql endpoint that returns a single member role. The member role is searchable by id.

Graphql

Request

{
  memberRole(id: "gid://gitlab/MemberRole/18") {
    id
    name
    readVulnerability
    readCode
    adminGroupMember
  }
}

Response

{
  "data": {
    "memberRole": {
      "id": "gid://gitlab/MemberRole/18",
      "name": "Role name",
      "readVulnerability": true,
      "readCode": true,
      "adminGroupMember": false
    }
  }
}

How to set up and validate locally

1. Create a member role, eg. using graphql
2. Get that member role using the provided graphql query

Database

• I use Custom Roles Root Group for queries as we test custom roles there and have at least few records

Query plans

• Filtering member roles by group
• Filtering member roles by id
• Filtering member roles by multiple ids

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 #428772 (closed)