feat: Add UnstructuredCSVLoader for CSV files (#5844)

### Summary

Adds an `UnstructuredCSVLoader` for loading CSVs. One advantage of using
`UnstructuredCSVLoader` relative to the standard `CSVLoader` is that if
you use `UnstructuredCSVLoader` in `"elements"` mode, an HTML
representation of the table will be available in the metadata.

#### Who can review?

@hwchase17
 @eyurtsev

tests/integration_tests/document_loaders/test_csv_loader.py

@@ -0,0 +1,15 @@
+import os
+from pathlib import Path
+
+from langchain.document_loaders import UnstructuredCSVLoader
+
+EXAMPLE_DIRECTORY = file_path = Path(__file__).parent.parent / "examples"
+
+
+def test_unstructured_csv_loader() -> None:
+    """Test unstructured loader."""
+    file_path = os.path.join(EXAMPLE_DIRECTORY, "stanley-cups.csv")
+    loader = UnstructuredCSVLoader(str(file_path))
+    docs = loader.load()
+
+    assert len(docs) == 1

tests/integration_tests/examples/stanley-cups.csv

@@ -0,0 +1,5 @@
+Stanley Cups,,
+Team,Location,Stanley Cups
+Blues,STL,1
+Flyers,PHI,2
+Maple Leafs,TOR,13