change default sub domain after installation

Hello,

Cu needs to add external subdomain to the cluster, In OCP 4.2 It is not recommended to change the default sub-domain.

It was possible in RHOCP 3.11

• https://access.redhat.com/documentation/en-us/openshift_container_platform/3.11/html/configuring_clusters/setting-up-a-router#customizing-the-default-routing-subdomain

The requirement is to allow users to change default sub domain of the cluster after istallation adding the external sub domain as the default sub domain.