Enhancement: New WebFinger workflow

ownCloud Infinite Scale servers may provide a new WebFinger service which
clients can use to discover an actual instance they should connect to.

This is useful when a large organization operates multiple instances to
distribute the load on multiple servers, or single departments operate their
own servers, but they all use a common entrypoint URL and authentication
system.

https://github.com/owncloud/client/issues/10517
https://github.com/owncloud/client/pull/10665
https://owncloud.dev/services/webfinger/
