WeaviateHybridSearchRetriever option to enable scores. (#7861)

Description: This PR adds the option to retrieve scores and explanations
in the WeaviateHybridSearchRetriever. This feature improves the
usability of the retriever by allowing users to understand the scoring
logic behind the search results and further refine their search queries.

Issue: This PR is a solution to the issue #7855 
Dependencies: This PR does not introduce any new dependencies.

Tag maintainer: @rlancemartin, @eyurtsev

I have included a unit test for the added feature, ensuring that it
retrieves scores and explanations correctly. I have also included an
example notebook demonstrating its use.

tests/integration_tests/retrievers/test_weaviate_hybrid_search.py

@@ -61,6 +61,29 @@ class TestWeaviateHybridSearchRetriever:
             Document(page_content="bar", metadata={"page": 1}),
         ]
 
+    @pytest.mark.vcr(ignore_localhost=True)
+    def test_get_relevant_documents_with_score(self, weaviate_url: str) -> None:
+        """Test end to end construction and MRR search."""
+        texts = ["foo", "bar", "baz"]
+        metadatas = [{"page": i} for i in range(len(texts))]
+
+        client = Client(weaviate_url)
+
+        retriever = WeaviateHybridSearchRetriever(
+            client=client,
+            index_name=f"LangChain_{uuid4().hex}",
+            text_key="text",
+            attributes=["page"],
+        )
+        for i, text in enumerate(texts):
+            retriever.add_documents(
+                [Document(page_content=text, metadata=metadatas[i])]
+            )
+
+        output = retriever.get_relevant_documents("foo", score=True)
+        for doc in output:
+            assert "_additional" in doc.metadata
+
     @pytest.mark.vcr(ignore_localhost=True)
     def test_get_relevant_documents_with_filter(self, weaviate_url: str) -> None:
         """Test end to end construction and MRR search."""