community[patch]: add attribution_token within GoogleVertexAISearchRetriever (#18520)

- **Description:** Add attribution_token within
GoogleVertexAISearchRetriever so user can provide this information to
Google support team or product team during debug session.
    
Reference:
https://cloud.google.com/generative-ai-app-builder/docs/view-analytics#user-events

Attribution tokens. Attribution tokens are unique IDs generated by
Vertex AI Search and returned with each search request. Make sure to
include that attribution token as UserEvent.attributionToken with any
user events resulting from a search. This is needed to identify if a
search is served by the API. Only user events with a Google-generated
attribution token are used to compute metrics.
    
    - **Issue:** No
    - **Dependencies:** No
    - **Twitter handle:** abehsu1992626
---------

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

libs/community/langchain_community/retrievers/google_vertex_ai_search.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any, Dict, List, Optional, Sequence
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, Sequence, Tuple
 
 from langchain_core.callbacks import CallbackManagerForRetrieverRun
 from langchain_core.documents import Document
@@ -345,6 +345,11 @@ class GoogleVertexAISearchRetriever(BaseRetriever, _BaseGoogleVertexAISearchRetr
         self, query: str, *, run_manager: CallbackManagerForRetrieverRun
     ) -> List[Document]:
         """Get documents relevant for a query."""
+        return self.get_relevant_documents_with_response(query)[0]
+
+    def get_relevant_documents_with_response(
+        self, query: str
+    ) -> Tuple[List[Document], Any]:
         from google.api_core.exceptions import InvalidArgument
 
         search_request = self._create_search_request(query)
@@ -382,7 +387,7 @@ class GoogleVertexAISearchRetriever(BaseRetriever, _BaseGoogleVertexAISearchRetr
                 + f" Got {self.engine_data_type}"
             )
 
-        return documents
+        return documents, response
 
 
 class GoogleVertexAIMultiTurnSearchRetriever(