The role of subnetwork. Currently, this field is only used when
purpose = INTERNAL_HTTPS_LOAD_BALANCER. The value can be set to
ACTIVE or BACKUP. An ACTIVE subnetwork is one that is currently
being used for Internal HTTP(S) Load Balancing. A BACKUP subnetwork
is one that is ready to be promoted to ACTIVE or is currently
draining. This field can be updated with a patch request.