Improve serializer typing

[khvn26]

Currently, in order to use serializer classes imported from flagsmith-common in strictly typed codebases, we have to use type: ignore or cast created/updated instances to target model types.

We should look into the following:

1. Add a TypeVar to SerializerWithMetadata so subclasses can be annotated as e.g. class FeatureSerializerWithMetadata(SerializerWithMetadata[Feature], CreateFeatureSerializer in the consuming code.
2. In common.types module, try to import the Core API models under if TYPE_CHECKING, and only use type aliases to model.Model if they are not available.

[khvn26]

Is this still relevant? Seems like the only untyped serializer left is CreateSegmentOverrideFeatureStateSerializer because of the drf_writable_nested dependency?

CC @emyller

[emyller]

Seems like the only untyped serializer left is CreateSegmentOverrideFeatureStateSerializer because of the drf_writable_nested dependency?

That might be true in flagsmith-common now that SegmentSerializer and SerializerWithMetadata were moved to core. I think the metadata one could see typing improvements separately. Can we perhaps move this issue to Flagsmith/flagsmith and tackle that? I can also give it a try while working on Flagsmith/flagsmith#5825.