[Feature] FUSE Client

It would be very cool to be able to mount the Docspell instance as [FUSE](https://en.wikipedia.org/wiki/Filesystem_in_Userspace) mount to be able to browse the content of Docspell directly in the filesystem. It would be enough to be read-only, no need to be able to upload files.

One use-case is e.g. in CH when one has to do the "Steuererklärung" a lot of documents have to be collected and finally be uploaded to the "Steueramt". Currently that means all the files in Docspell have to be manually downloaded and then uploaded again. With FUSE one could simply browse to the files like they are stored locally, without downloading first.