Support for complex order_by in Elasticsearch/OpenSearch

The Elasticsearch and OpenSearch backends currently don't support [`order_by` expressions](https://docs.djangoproject.com/en/5.2/ref/models/expressions/#using-f-to-sort-null-values) such as `F("field_name").asc(nulls_first=True)`


See: https://github.com/wagtail/wagtail/pull/12965