Draft: Add GraphQL fields for member role users

What does this MR do and why?

Example query for custom roles:

query memberRole {
  memberRole(id: "gid://gitlab/MemberRole/3") {
    name
    roleUsers {
      nodes {
        user {
          name
        }
        groupsCount
        projectsCount
        members {
          nodes {
            fullName
          }
        }
      }
    }
  }
}

Example response for custom roles:

{
  "data": {
    "memberRole": {
      "name": "Test",
      "roleUsers": {
        "nodes": [
          {
            "user": {
              "name": "Eleanore Berge"
            },
            "groupsCount": 0,
            "projectsCount": 1,
            "members": {
              "nodes": [
                {
                  "fullName": "Gitlab Org / Gitlab Test"
                }
              ]
            }
          },
          {
            "user": {
              "name": "Aleshia Gutmann"
            },
            "groupsCount": 1,
            "projectsCount": 1,
            "members": {
              "nodes": [
                {
                  "fullName": "Gitlab Org / Gitlab Test"
                },
                {
                  "fullName": "Flightjs"
                }
              ]
            }
          },
          {
            "user": {
              "name": "John Doe"
            },
            "groupsCount": 1,
            "projectsCount": 0,
            "members": {
              "nodes": [
                {
                  "fullName": "Security Group"
                }
              ]
            }
          }
        ]
      }
    }
  }
}

Example query for default roles:

query {
  standardRoles(accessLevel: 30) {
    nodes {
      name
      roleUsers(search: "car") {
        nodes {
          groupsCount
          projectsCount
          user {
            name
          }
          members {
            nodes {
              fullName
            }
          }
        }
      }
    }
  }
}

Example response for default roles:

{
  "data": {
    "standardRoles": {
      "nodes": [
        {
          "name": "Developer",
          "roleUsers": {
            "nodes": [
              {
                "groupsCount": 0,
                "projectsCount": 1,
                "user": {
                  "name": "Aleshia Gutmann"
                },
                "members": {
                  "nodes": [
                    {
                      "fullName": "Jashkenas / Underscore"
                    }
                  ]
                }
              }
            ]
          }
        }
      ]
    }
  }
}

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Related to #489771