For Groups and Project Members API - add public_email to user information response

When a user is returned from the Group and Project Members API, it does not include the public_email field.

API documentation: https://docs.gitlab.com/12.10/ee/api/members.html

{
    "id": 2,
    "username": "john_doe",
    "name": "John Doe",
    "state": "active",
    "avatar_url": "https://www.gravatar.com/avatar/c2525a7f58ae3776070e44c106c48e15?s=80&d=identicon",
    "web_url": "http://192.168.1.8:3000/root",
    "expires_at": "2012-10-22T14:13:35Z",
    "access_level": 30,
    "group_saml_identity": {
      "extern_uid":"ABC-1234567890",
      "provider": "group_saml",
      "saml_provider_id": 10
    }
  }

We should add this information to help customers who use this API call to find out about the users associated with groups and projects.