source-stripe-native: improve typing

Typing has been improved by extracting common attributes for each model into shared classes, like `BaseStripeObject`, then using those classes within `api.py`. I also did small refactors that improve typing and readability.

Discover snapshot changes are all expected due to the new, shared classes that contain common attributes & minor description changes. Noteably, all non-child streams have a `created` field in their documents, and that field is included in the model since we rely on it during backfills.