Support similarity_score_threshold retrieval with Chroma (#5655)

Fixes https://github.com/hwchase17/langchain/issues/5067

Verified the following code now works correctly:
```
db = Chroma(persist_directory=index_directory(index_name), embedding_function=embeddings)
retriever = db.as_retriever(search_type="similarity_score_threshold", search_kwargs={"score_threshold": 0.4})
docs = retriever.get_relevant_documents(query)
```

langchain/vectorstores/chroma.py

@@ -232,6 +232,14 @@ class Chroma(VectorStore):
 
         return _results_to_docs_and_scores(results)
 
+    def _similarity_search_with_relevance_scores(
+        self,
+        query: str,
+        k: int = 4,
+        **kwargs: Any,
+    ) -> List[Tuple[Document, float]]:
+        return self.similarity_search_with_score(query, k)
+
     def max_marginal_relevance_search_by_vector(
         self,
         embedding: List[float],