community: Added Lantern as VectorStore (#12951)

Support [Lantern](https://github.com/lanterndata/lantern) as a new
VectorStore type.

- Added Lantern as VectorStore.
It will support 3 distance functions `l2 squared`, `cosine` and
`hamming` and will use `HNSW` index.
- Added tests
- Added example notebook

libs/langchain/tests/unit_tests/indexes/test_indexing.py

@@ -1250,5 +1250,6 @@ def test_compatible_vectorstore_documentation() -> None:
         "VespaStore",
         "Weaviate",
         "ZepVectorStore",
+        "Lantern",
     }
     assert compatible == documented