Add FTP support to jobstores

[stxue1]

Closes #5134

Also turns all url_exists requests and size requests to HEAD requests instead, as mentioned here: https://github.com/DataBiosphere/toil/issues/5114#issuecomment-2392527695

In my local testing, this significantly improves FTP performance. (Previously an existence check on a small text file on an insecure FTP server took around 5 seconds, though I'm unsure why it would take so long originally)

Changelog Entry

To be copied to the draft changelog by merger:

• Added proper FTP support for jobstores
• URL existence and size gets/checks are now done with HEAD requests

Reviewer Checklist

• [ ] Make sure it is coming from issues/XXXX-fix-the-thing in the Toil repo, or from an external repo.
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Merger Checklist

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