feat: Add extract_attachment_info

[mallorih]

This is a draft PR until the previous PR is merged.

This PR adds the ability to extract attachment information and payload from an email and save it to disk or store in a python object.

Testing

 import email
  from unstructured.partition.email import extract_attachment_info

  with open("example-docs/fake-email-attachment.eml", "r") as f:
      msg = email.message_from_file(f)
  attachment_info = extract_attachment_info(msg, output_dir="example-docs")

[MthwRobinson]

Implementation looks good! Just one naming suggestion. Can we also add a section to the docs showing how to deal with attachments?

[MthwRobinson]

Looks good! LGTM after the following changes. Also made one small docs suggestion. Feel free to merge without another review once these items are in:

• Remove DRAFT from the PR title
• Add a testing section to the PR summary with test code (can be the same snippet from the docs)
• Run make tidy to fix the lint job