Add Support for OpenSearch Vector database (#1191)

### Description
This PR adds a wrapper which adds support for the OpenSearch vector
database. Using opensearch-py client we are ingesting the embeddings of
given text into opensearch cluster using Bulk API. We can perform the
`similarity_search` on the index using the 3 popular searching methods
of OpenSearch k-NN plugin:

- `Approximate k-NN Search` use approximate nearest neighbor (ANN)
algorithms from the [nmslib](https://github.com/nmslib/nmslib),
[faiss](https://github.com/facebookresearch/faiss), and
[Lucene](https://lucene.apache.org/) libraries to power k-NN search.
- `Script Scoring` extends OpenSearch’s script scoring functionality to
execute a brute force, exact k-NN search.
- `Painless Scripting` adds the distance functions as painless
extensions that can be used in more complex combinations. Also, supports
brute force, exact k-NN search like Script Scoring.

### Issues Resolved 
https://github.com/hwchase17/langchain/issues/1054

---------

Signed-off-by: Naveen Tatikonda <navtat@amazon.com>

docs/ecosystem/opensearch.md

@@ -0,0 +1,21 @@
+# OpenSearch
+
+This page covers how to use the OpenSearch ecosystem within LangChain.
+It is broken into two parts: installation and setup, and then references to specific OpenSearch wrappers.
+
+## Installation and Setup
+- Install the Python package with `pip install opensearch-py`
+## Wrappers
+
+### VectorStore
+
+There exists a wrapper around OpenSearch vector databases, allowing you to use it as a vectorstore 
+for semantic search using approximate vector search powered by lucene, nmslib and faiss engines 
+or using painless scripting and script scoring functions for bruteforce vector search.
+
+To import this vectorstore:
+```python
+from langchain.vectorstores import OpenSearchVectorSearch
+```
+
+For a more detailed walkthrough of the OpenSearch wrapper, see [this notebook](../modules/indexes/vectorstore_examples/opensearch.ipynb)