feat: add url kwarg to partititon

[MthwRobinson]

Summary

Closes #464 and addresses user requests in langchain#979 Adds a url kwarg to partition to allow users to process remote documents directly. If the user passes in content_type, partition will process the document using content_type as the MIME type. Otherwise, it will use the Content-Type header to determine the MIME type.

Testing

  from unstructured.partition.auto import partition

  # A markdown file
  url = "https://raw.githubusercontent.com/Unstructured-IO/unstructured/main/LICENSE.md"
  elements = partition(url=url, content_type="text/markdown")

  # An HTML file
  url = "https://www.understandingwar.org/backgrounder/russian-offensive-campaign-assessment-april-11-2023"
  elements = partition(url=url)

  # A PDF file
  url = "https://www.understandingwar.org/sites/default/files/Russian%20Offensive%20Campaign%20Assessment%2C%20April%2011%2C%202023.pdf"
  elements = partition(url=url, strategy="fast")

[qued]

LGTM!