Add support for backend type and instance group backend

[riupie]

When we create network load balancer in GCP, we can choose:

• Single region or Multi region
• Backend type between target pool/instance and backend service (for now it only support target)

But it seems this module doesn't yet has support for this feature. It would be good if you guys can add this feature in future release.

Thanks.

[betsy-lichtenberg]

Adding some more info to this issue.

The classic "Network Load Balancer" (aka external TCP/UDP) has target pool backends.

The new external TCP/UDP load balancer has backends that use backend services instead of target pools.
Here's a document that discusses the two architectures (target pool vs backend service):
https://cloud.google.com/load-balancing/docs/network#architecture

Here's more conceptual info about the backend service-based architecture:
https://cloud.google.com/load-balancing/docs/network/networklb-backend-service

Here's an example that uses a backend service:
https://cloud.google.com/load-balancing/docs/network/setting-up-network-backend-service

Here's a document that shows how to migrate a target pool-based LB to a backend service-based LB:
https://cloud.google.com/load-balancing/docs/network/transition-to-backend-services

[github-actions]

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