Declare Prefetch as generic and do specialization in the plugin

[UnknownPlatypus]

I have made things!

Followup to #2779 to handle more Prefetch usages using a generic class over the prefetched queryset and to_attr value (with plugin specialization to LiteralString).

We now:

• handle calls defined outside of prefetch_related calls
• handle Prefetch classes created from external function or variable assignments
• correctly infer for case with the to_attr attribute filled but not the queryset attribute.

reveal_type(
    Article.objects.prefetch_related(
-     models.Prefetch("tags", to_attr="tags")).get().tags)  # N: Revealed type is "builtins.list[Any]"
+     models.Prefetch("tags", to_attr="tags")).get().tags)  # N: Revealed type is "builtins.list[myapp.models.Tag]"
    )
)

I've also expanded the tests with a test for annotated prefetched queryset

[sobolevn]

Thanks again!