feat: expose WebhooksList and JsonSerializable from public types module (#800)

## Summary

- Move `WebhooksList` and `JsonSerializable` from the private
`_internal_types.py` to the public `apify_client.types` module so users
writing custom HTTP clients or consuming public method signatures can
import them.
- Update all internal importers to the new location and drop the
now-empty `_internal_types.py` (its only remaining alias,
`TerminalActorJobStatus`, had zero importers — the actually-used
`_TerminalActorJobStatus` lives privately in
`_resource_clients/_resource_client.py`).