Add Cross Navigation Enrichment

[spenes]

This enrichment will be able to parse the extended cross navigation format in _sp querystring parameter and attach the cross_navigation context to an event.

The extended cross navigation format can be described by _sp={domainUserId}.{timestamp}.{sessionId}.{subjectUserId}.{sourceId}.{platform}.{reason}

The changes in the trackers to allow this extended format can be seen in:

• Android tracker
• JavaScript tracker

The corresponding iglu-central pull requests are:

• to add the cross_navigation context schema
• to add the cross_navigation_config enrichment schema

cc @adatzer @matus-tomlein