feat: add UnstructuredExcelLoader for .xlsx and .xls files (#5617)

# Unstructured Excel Loader

Adds an `UnstructuredExcelLoader` class for `.xlsx` and `.xls` files.
Works with `unstructured>=0.6.7`. A plain text representation of the
Excel file will be available under the `page_content` attribute in the
doc. If you use the loader in `"elements"` mode, an HTML representation
of the Excel file will be available under the `text_as_html` metadata
key. Each sheet in the Excel document is its own document.

### Testing

```python
from langchain.document_loaders import UnstructuredExcelLoader

loader = UnstructuredExcelLoader(
    "example_data/stanley-cups.xlsx",
    mode="elements"
)
docs = loader.load()
```

## Who can review?

@hwchase17
@eyurtsev

tests/integration_tests/document_loaders/test_excel.py

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