Explicit Filter/Sort Definitions

Currently, all attrs are filterable and sortable.  Should be a way to whitelist/blacklist attrs.

Currently, considering blacklisting a filter on the attr:

```csharp
public class MyEntity : Identifiable {
  [Attr("name", canFilter: false, canSort: false)]
  public string Name { get; set; }
}
```

Open to API suggestions.