Allow handling instance custom roles via graphql

What does this MR do and why?

It allows handling instance-level custom roles via GraphQL.

Related MR: !139357 (closed)

How to set up and validate locally

• test handling both group custom roles (you have to be a group owner) and instance level custom roles (you have to be an admin)
• instance-level custom roles should work only on self-managed
• group-level custom roles should work only on SaaS (check Simulate a SaaS instance guide)

GraphQL queries to test

Create

mutation {
  memberRoleCreate(input: {
    groupPath: "flightjs"
    name: "Role name", 
    description: "This is a new test role",
    readCode: true,
    readVulnerability: true,
    baseAccessLevel: GUEST
  }) {
    errors
    memberRole {
      id
      name
    }
  }
}

Update

mutation {
  memberRoleUpdate(input: {
    id: "gid://gitlab/MemberRole/11", 
    name: "New name", 
    description: "This is a test role new"
  }) {
    errors
    memberRole {
      id
      name
      description
    }
  }
}

Delete

mutation {
  memberRoleDelete(input: {
    id: "gid://gitlab/MemberRole/11"
  }) {
    errors
    memberRole {
      id
      name
      description
    }
  }
}

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 #429278 (closed)