backend: enforce backend HTTP requests make progress

[MichaelEischer]

What does this PR change? What problem does it solve?

Implements http.Request based timeouts for connections proposed in https://github.com/restic/restic/issues/4193#issuecomment-2067988727 .

Was the change previously discussed in an issue or on the forum?

Fixes #4193. Part of https://github.com/restic/restic/issues/4627 . Depends on #4605. Replaces https://github.com/restic/restic/pull/4778

Checklist

• [x] I have read the contribution guidelines.
• [x] I have enabled maintainer edits.
• [x] I have added tests for all code changes.
• ~~[ ] I have added documentation for relevant changes (in the manual).~~
• [x] There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• [x] I have run gofmt on the code in all commits.
• [x] All commit messages are formatted in the same style as the other commits in the repo.
• [x] I'm done! This pull request is ready for review.