add initial wsdiscovery support

Summary:
needs kdsoap >= 1.8.50 (current master).
builds a static variant of kdsoap-ws-discovery-client but can also
use system's version if available.

also reinvents how discovery works:
there are now Discoverers for dnssd and wsd that get started/stopped.
the Discoverers discover servers and emit them as Discoveries. Discoveries
are then converted to udsentries for kio.

BUG: 392447
FIXED-IN: 20.04.0

Test Plan:
discover all the things!

- windows 10 test vm with remote address
- https://github.com/christgau/wsdd on remote host
- avahi smb on remote host

Reviewers: dfaure, #frameworks, #dolphin, ngraham

Reviewed By: dfaure, ngraham

Subscribers: meven, schmeisser, bcooksley, ngraham, caspermeijn, davidedmundson, kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D25682