Add webdav-rfc4918 to known WebDAV implementations

[manfred-kaiser]

Adds webdav-rfc4918 to the known WebDAV implementations and edits the recommended webdav package.

webdav4 has no entry point and no register_implementation() call of its own, so this hardcoded dict entry is the only way to use webdav4 as fsspec backend and is not touched for backwards compatibility.

The err hint only fires when that import fails, i.e. when webdav-rfc-4918 or webdav4 isn't installed.

It now lists both packages, with webdav-rfc4918 marked recommended because it passes the full fsspec conformance suite.

webdav4 passes 24 of 137 tests.

Anyone who already has either package installed sees no change:

• with webdav4 installed, the import succeeds and the hint never fires
• with webdav-rfc4918 installed, it is loaded via the defined entry point

Note: if both are installed, webdav-rfc4918 is used because the entry point has precedence over the default dict.

[martindurant]

@alvarobartt , any issues with this?