Offer anonymous connection to read-only WebDav server

[jeremypw]

What Happened

Set up a WebDav server with unauthenticated read-only access. Attempted to connect to this with the ConnectServer dialog. Could not connect until a user name and password was entered - the user had to be registered locally.
The server could be connected to by entering the address dav://<server-name>/webdav into Files pathbar.

Expected Behavior

An option to attempt anonymous connection should be offered as it is with FTP.
Rather than adding a further option to the combobox, radio buttons for "anonymous" and "authenticated" could be offered.

Platform Information

Elementary Odin daily

[queeup]

Same here for ssh: #1666 (kindoff)

[jeremypw]

It is possible to connect to a suitable WebDAV server by entering a random user name and no password so this issue is about having an option in the dropdown that does that automatically which is easy enough.