Support remote by external executable

I've bumped into this issue a couple of times, and I'd like to second @ivandeex's initiative on supporting encryption systems by dedicated teams.

Encryption is pretty complex, after all, so it requires a lot of work to get it right and future-proof. Why not use tools by others to make rclone an already awesome piece of software even more awesome. I can see #1647 and #651 being discussed already.

Let me add another two cents!

I think, the idea can be generalised! Why not have external hooks in general, which could then call external programs to provide (part of) the chain of wrapped remotes?

• rclone would have a core remote, mounted (if permitted) or even just served over WebDAV (on things like Termux)
• If that succeeds, another external remote would spin up cryptomator-cli (or any similar thing, like cryfs, tomb, etc.),
• And finally the topmost remote would wrap that so it's completely transparent. (So, sync would work for example)

Every such external step could then have an input parameter for the underlying path, and one output parameter for the path they provide. (Path being understood more like an URL here; so http://localhost:12345/webav would work) It would be up to the external executable to parse such an URL)

Perhaps additionally it would be worth to check the external tools exit status.