Fix exporting GCP Vertex Matching Engine from vectorstores (#5793)

The Vertex Matching Engine docs include [the
line](b177a29d3f/docs/modules/indexes/vectorstores/examples/matchingengine.ipynb?short_path=54ebfde#L32)
`from langchain.vectorstores import MatchingEngine` which doesn't work
as it wasn't added to the vectorestores module exports.



  - @dev2049

langchain/vectorstores/__init__.py

@@ -10,6 +10,7 @@ from langchain.vectorstores.docarray import DocArrayHnswSearch, DocArrayInMemory
 from langchain.vectorstores.elastic_vector_search import ElasticVectorSearch
 from langchain.vectorstores.faiss import FAISS
 from langchain.vectorstores.lancedb import LanceDB
+from langchain.vectorstores.matching_engine import MatchingEngine
 from langchain.vectorstores.milvus import Milvus
 from langchain.vectorstores.mongodb_atlas import MongoDBAtlasVectorSearch
 from langchain.vectorstores.myscale import MyScale, MyScaleSettings
@@ -56,4 +57,5 @@ __all__ = [
     "Clickhouse",
     "ClickhouseSettings",
     "Tigris",
+    "MatchingEngine",
 ]