Proposal: add virtual archive access via files_external

[rotdrop]

It would be somehow cool to add a virtual archive access by providing a Storage-implementation and just mount the archive-storage instead of really extracting it. This would be a bit in the spirit of desktop archive-apps which provide you with a "virtual" view into the archive file and additionally offer the possibility to extract the thing somewhere.

Perhaps it would also be nice to install a sidebar tab with further archive information and actions, like

• uncompressed archive size
• extract here or there
• mount / unmount the archive file

[rotdrop]

I have to admit that I could not resist to implement my own archive handling app which actual does just this:

https://github.com/rotdrop/nextcloud-app-files-archive

Sorry for that. Normally it would have been better to bundle such efforts, but as there was little response here to pull requests and issues I just could not resist.