Add support for full QuerySet API

As of right now, only basic QuerySet methods are supported for EAV values, namely:
`filter`, `exclude` and `get`. Therefore, something like:

```
Patient.objects.all().order_by('eav__date_of_admission')
```
will not work.

It would be useful to add support for the full QuerySet API.

Start with the EAV query entry point represented by Patient.objects and compare the requested behavior with Django's QuerySet API documentation. Define which QuerySet methods must support EAV values, beginning with the shown order_by('eav__date_of_admission') example, and verify that the selected API operations work consistently.

Written by the indexing model from the issue text.