PR for mount support on windows

I’ve opened a PR to support mount on windows: https://github.com/restic/restic/pull/2862

The PR gives an overview on how to use it. The design and integration into restic might need some changes. We probably need a common snapshot access layer that can be used by different subsystems (mount on unix like/mount on windows/webdav/etc.)

I hope to get some feedback soon.