Fix ipv6 and align loadbalancer address types

[wiktorn]

Follow up fix for #2879 (.IP Version and IP Address cannot be specified at the same time)

API returns:

Error: Error creating GlobalForwardingRule: googleapi: Error 400: Invalid value for field \'resource.ipVersion\': \'IPV4\'. IP Version and IP Address cannot be specified at the same time., invalid

  with module.glb-test-0.google_compute_global_forwarding_rule.default[""],
  on fabric/modules/net-lb-app-ext/main.tf line 44, in resource "google_compute_global_forwarding_rule" "default":
  44: resource "google_compute_global_forwarding_rule" "default" {

Add support for __missing__ when testing for plan values.

---

Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

Upgrade Notes

modules/net-lb-ext: forwarding_rules_config.ip_version was renamed to forwarding_rules_config.ipv6 and its type is now bool
modules/net-lb-int: forwarding_rules_config.ip_version was renamed to forwarding_rules_config.ipv6 and its type is now bool