Add plugin to send requests with user ID to a Kantar FocalMeter endpoint

[matus-tomlein]

This will integrate the tracker with the FocalMeter system by Kantar that measures audience of content through a router meter.

The tracker should make a request with the domain user ID whenever that changes. This will be implemented through a plugin that will check the ID in the events in the afterTrack callback. The URL endpoint to send the requests to should be configurable when initializing the plugin.