The purpose of the resource. This field can be either
PRIVATE_RFC_1918 or INTERNAL_HTTPS_LOAD_BALANCER. A subnetwork with
purpose set to INTERNAL_HTTPS_LOAD_BALANCER is a user-created
subnetwork that is reserved for Internal HTTP(S) Load Balancing. If
unspecified, the purpose defaults to PRIVATE_RFC_1918. The
enableFlowLogs field isn't supported with the purpose field set to
INTERNAL_HTTPS_LOAD_BALANCER.