webdav: add a test for digest authentication - #9875

What does this change do?

Adds Digest authentication to the WebDAV backend.

When a request comes back 401 with a digest challenge
rclone keeps the challenge, retries signed, and signs the rest of the session.
Signing is done in a RoundTripper wrapping rclone's transport.

Supersedes #8273 and does the two things @ncw asked for in review there: detect
digest from the 401 instead of probing at startup, and keep rclone's transport
instead of building a new client.

Opening early for feedback on the approach.

Linked issue

Fixes #2110

Related: #8273

For new or changed backends

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
• I have read the contribution guidelines.
• (If I used AI tools to help write this code) I have read and understood the AI-assisted contributions guidance, and I have tested and take ownership of this change myself.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
• All commit messages are in house style.
• (Backend changes only) test_all passes for this backend and if submitting a new backend can provide a test account for the integration tester - see CONTRIBUTING.md.
• This Pull Request is ready for review.