URL chaining for fsspec

[normanrz]

Since there is already a http implementation in fsspec, I think it would be handy to implement URL chaining for webdav4.
That would allow something like this:

import fsspec

fsspec.open("webdav4::https://example.com")

With proper backend registration, webdav4 would then be called by fsspec and could handle the URL as it pleases. It would not need to pass control further to HTTPFileSystem.

I'd be happy to help with a PR.