Operator feature: orchestrate link groups by cluster horizon

[qrkourier]

The new router link groups feature enables a topology like this, spanning the fabric across clusters via their Ingress to Ziti routers that are dual-homed in both a public and cluster-internal link group.

The result is a resilient, efficient Ziti fabric inside each cluster, probably one router per node, and a scaled set of "public" routers that are advertising two link listeners each: one public, one cluster-internal.