Support read-only properties on entity types

[natemcmaster]

Example: produce fields for totalCount and completedCount on user

    public class User : Node
    {
        public ICollection<Todo> Todos { get; set; }

        public int TotalCount => Todos.Count;
        public int CompletedCount => Todos.Count(t => t.Complete);
    }

    public class Todo : Node
    {
        public int UserId { get; set; }
        public User User { get; set; }
        public string Text { get; set; }
        public bool Complete { get; set; }
    }