Add better typing to support variable OrderStatus objects

[philvarner]

What I'm changing

A existing problem with the typing for various methods is that an implementer could not create their own implementation of OrderStatus and have additional properties in it be serialized in server responses. This is due to fastapi's use of type annotations for determining what serializer to use when returning an object from an endpoint. As defined, any object returned as the status (either with an Order or from the Order statuses endpoint) will be serialized as an Order Status, so any additional provider-specific attributes are not included. However, by adding the correct generic typing, a provider can redefine these endpoints so their own custom OrderStatus class will be serialized correctly.

How I did it

1. Refactored the Callable aliases GetOrders, GetOrder, GetOrderStatuses as classes that implement dunder call, so that they can have generic parameters
2. Refactor the uses of RootRouter in ProductRouter to have two protocols (ConformancesSupport and RootProvider), to decouple the actual (generified) RootRouter from the uses in that class.

Checklist

• Tests pass: ./scripts/run-tests.sh
• Checks pass: uv run pre-commit run --all-files
• CHANGELOG is updated (if necessary)