Airbyte loaders: Fix last_state getter (#9314)

This PR fixes the Airbyte loaders when doing incremental syncs. The
notebooks are calling out to access `loader.last_state` to get the
current state of incremental syncs, but this didn't work due to a
refactoring of how the loaders are structured internally in the original
PR.

This PR fixes the issue by adding a `last_state` property that forwards
the state correctly from the CDK adapter.

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/document_loaders/airbyte.py

@@ -60,6 +60,10 @@ class AirbyteCDKLoader(BaseLoader):
             stream_name=self._stream_name, state=self._state
         )
 
+    @property
+    def last_state(self) -> Any:
+        return self._integration.last_state
+
 
 class AirbyteHubspotLoader(AirbyteCDKLoader):
     """Load from `Hubspot` using an `Airbyte` source connector."""