enhancement: support headers for non-html urls (#3166)
### Summary
Updates the `UnstructuredURLLoader` to support passing in headers for
non HTML content types. While this update maintains backward
compatibility with older versions of `unstructured`, we strongly
recommended upgrading to `unstructured>=0.5.13` if you are using the
`UnstructuredURLLoader`.
### Testing
#### With headers
```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])
```
#### Without headers
```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, strategy="fast")
docs = loader.load()
print(docs[0].page_content[:1000])
```
---------
Co-authored-by: Zander Chase <130414180+vowelparrot@users.noreply.github.com>
