Add `WebDAV` backend

[james-pre]

Recently I took a look at WebDAV. It looks like a place ZenFS could really be useful as WebDAV is designed around editing server-side files. While the existing Fetch backend allows this, WebDAV is another way to do something similar and it would be great if ZenFS could support it.

Being an extension of HTTP, this can be done almost the same as Fetch and will be fully cross-platform.

[james-pre]

WebDAV's use of XML means that a dependency has to be pulled in for cross- platform support. It may be worth looking into whether JSON can be the main metadata format, though this would be a custom protocol (so, not WebDAV).