add internal L7 LB submodule

[naseemkullah]

https://cloud.google.com/load-balancing/docs/l7-internal

[naseemkullah]

Related: terraform-google-modules/terraform-google-lb-internal#38

[github-actions]

This issue is stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 7 days

[morgante]

Example: https://github.com/GoogleCloudPlatform/professional-services/tree/main/examples/terraform-ilb