Add configuration to send requests with user ID to a Focal Meter endpoint (close #571)

[matus-tomlein]

Issue #571

Similar to PR #572 which implemented the FocalMeter configuration for the version 4, this PR implements the configuration for version 5 of the tracker. In contrast with the v4 implementation, this one makes use of the new plugin API and the afterTrack callback. The public API is the same as for v4.

Documentation: Sending session user identifier to Kantar FocalMeter

The tracker has the ability to send the user identifier (userId present in the session context) to a Kantar FocalMeter endpoint. This integration enables measuring the audience of content through the FocalMeter router meter.

To enable this feature, you can pass the FocalMeterConfiguration configuration with the URL of the Kantar endpoint. For example:

FocalMeterConfiguration focalMeterConfiguration = new FocalMeterConfiguration("https://thekantarendpoint.com");
Snowplow.createTracker(getApplicationContext(),
        namespace,
        networkConfiguration,
        focalMeterConfiguration