Add custom vector fields and text fields for OpenSearch (#2652)

**Description**
Add custom vector field name and text field name while indexing and
querying for OpenSearch

**Issues**
https://github.com/hwchase17/langchain/issues/2500

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

tests/integration_tests/vectorstores/test_opensearch.py

@@ -23,6 +23,30 @@ def test_opensearch() -> None:
     assert output == [Document(page_content="foo")]
 
 
+def test_opensearch_with_custom_field_name() -> None:
+    """Test indexing and search using custom vector field and text field name."""
+    docsearch = OpenSearchVectorSearch.from_texts(
+        texts,
+        FakeEmbeddings(),
+        opensearch_url=DEFAULT_OPENSEARCH_URL,
+        vector_field="my_vector",
+        text_field="custom_text",
+    )
+    output = docsearch.similarity_search(
+        "foo", k=1, vector_field="my_vector", text_field="custom_text"
+    )
+    assert output == [Document(page_content="foo")]
+
+    text_input = ["test", "add", "text", "method"]
+    OpenSearchVectorSearch.add_texts(
+        docsearch, text_input, vector_field="my_vector", text_field="custom_text"
+    )
+    output = docsearch.similarity_search(
+        "add", k=1, vector_field="my_vector", text_field="custom_text"
+    )
+    assert output == [Document(page_content="foo")]
+
+
 def test_opensearch_with_metadatas() -> None:
     """Test end to end indexing and search with metadata."""
     metadatas = [{"page": i} for i in range(len(texts))]