feat: Add UnstructuredXMLLoader for .xml files (#5955)

# Unstructured XML Loader
Adds an `UnstructuredXMLLoader` class for .xml files. Works with
unstructured>=0.6.7. A plain text representation of the text with the
XML tags will be available under the `page_content` attribute in the
doc.

### Testing
```python
from langchain.document_loaders import UnstructuredXMLLoader

loader = UnstructuredXMLLoader(
    "example_data/factbook.xml",
)
docs = loader.load()
```


## Who can review?

@hwchase17 
@eyurtsev

tests/integration_tests/document_loaders/test_xml.py

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