WIP: Add health endpoint

[Enrico204]

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

This pull request adds an endpoint named /health. It replies with:

• 200 OK when the repository base path is writable and there is some free space left
• 500 Internal Server Error when either the path is not writable by rest-server, or the disk is full

This endpoint can be used to monitor the status of a rest-server daemon (for example in k8s / docker)

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

Feature was requested and is being discussed here: https://github.com/restic/rest-server/issues/147

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

[Enrico204]

I added the external library https://github.com/itchio/ox . That library contains wrappers for many Windows syscall. I used some of them to get information about the repository path write permission for current user.

I had to call a Windows API directly, however I already made a pull request to the original library to add that wrapper.

I've tested the code on Windows and Linux.

Do I need to write some tests for this? If so, do you have any suggestion? Testing permissions and free space might be difficult...