enhancement: add elements mode to `UnstructuredURLLoader` (#3456)

### Summary

Updates the `UnstructuredURLLoader` to include a "elements" mode that
retains additional metadata from `unstructured`. This makes
`UnstructuredURLLoader` consistent with other unstructured loaders,
which also support "elements" mode. Patched mode into the existing
`UnstructuredURLLoader` class instead of inheriting from
`UnstructuredBaseLoader` because it significantly simplified the
implementation.

### Testing

This should still work and show the url in the source for the metadata

```python
from langchain.document_loaders import UnstructuredURLLoader

urls = ["https://www.understandingwar.org/sites/default/files/Russian%20Offensive%20Campaign%20Assessment%2C%20April%2011%2C%202023.pdf"]

loader = UnstructuredURLLoader(urls=urls, headers={"Accept": "application/json"}, strategy="fast")
docs = loader.load()
print(docs[0].page_content[:1000])
docs[0].metadata
``` 

This should now work and show additional metadata from `unstructured`.

This should still work and show the url in the source for the metadata

```python
from langchain.document_loaders import UnstructuredURLLoader

urls = ["https://www.understandingwar.org/sites/default/files/Russian%20Offensive%20Campaign%20Assessment%2C%20April%2011%2C%202023.pdf"]

loader = UnstructuredURLLoader(urls=urls, headers={"Accept": "application/json"}, strategy="fast", mode="elements")
docs = loader.load()
print(docs[0].page_content[:1000])
docs[0].metadata
```

langchain/document_loaders/url.py

@@ -15,6 +15,7 @@ class UnstructuredURLLoader(BaseLoader):
         self,
         urls: List[str],
         continue_on_failure: bool = True,
+        mode: str = "single",
         **unstructured_kwargs: Any,
     ):
         """Initialize with file path."""
@@ -29,6 +30,9 @@ class UnstructuredURLLoader(BaseLoader):
                 "`pip install unstructured`"
             )
 
+        self._validate_mode(mode)
+        self.mode = mode
+
         headers = unstructured_kwargs.pop("headers", {})
         if len(headers.keys()) != 0:
             warn_about_headers = False
@@ -48,6 +52,13 @@ class UnstructuredURLLoader(BaseLoader):
         self.headers = headers
         self.unstructured_kwargs = unstructured_kwargs
 
+    def _validate_mode(self, mode: str) -> None:
+        _valid_modes = {"single", "elements"}
+        if mode not in _valid_modes:
+            raise ValueError(
+                f"Got {mode} for `mode`, but should be one of `{_valid_modes}`"
+            )
+
     def __is_headers_available_for_html(self) -> bool:
         _unstructured_version = self.__version.split("-")[0]
         unstructured_version = tuple([int(x) for x in _unstructured_version.split(".")])
@@ -94,7 +105,15 @@ class UnstructuredURLLoader(BaseLoader):
                     continue
                 else:
                     raise e
-            text = "\n\n".join([str(el) for el in elements])
-            metadata = {"source": url}
-            docs.append(Document(page_content=text, metadata=metadata))
+
+            if self.mode == "single":
+                text = "\n\n".join([str(el) for el in elements])
+                metadata = {"source": url}
+                docs.append(Document(page_content=text, metadata=metadata))
+            elif self.mode == "elements":
+                for element in elements:
+                    metadata = element.metadata.to_dict()
+                    metadata["category"] = element.category
+                    docs.append(Document(page_content=str(element), metadata=metadata))
+
         return docs