Introduce support for regional health checks in the net-lb-app-int module while maintaining backward compatibility.
Added optional is_regional flag to health_check_configs (defaulting to false). When true, it creates google_compute_region_health_check instead of google_compute_health_check.
Updated backend services and outputs to merge both global and regional health check IDs. Added a new test case to verify regional health check functionality.
TAG=agy
CONV=6aff620c-e5a5-44eb-afe0-459cff820daa
* add service attachment support to lb app int module
* allow direct referencing of self managed ig in ilb module
* add service attachment support to net-ilb-int
* add service attachments example to net-lb-int
* fix resource name in net-lb-ext
* rename fwd rules resource in test inventories
* add toc to net-lb-int
