WIP: Authentication using TLS Client Certificates / mTLS

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What is the purpose of this change? What does it change?

Implements client certificate authentication / mTLS Especially the testfiles are WIP

Was the change discussed in an issue or in the forum before?

Closes #73

Checklist

• [x] I have enabled maintainer edits for this PR
• [ ] I have added tests for all changes in this PR
• [ ] I have added documentation for the changes (in the manual)
• [ ] There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• [ ] 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
• [ ] I'm done, this Pull Request is ready for review

[wojas]

PR https://github.com/restic/rest-server/pull/140 also implements TLS client auth with automatic certificate reload through https://github.com/PowerDNS/go-tlsconfig . I can rebase the PR in the next few weeks.

[berlincount]

Any movement expected on this PR? 🤔