Adding ula internal ipv6 support in compute_network and subnetwork

[modular-magician]

Fixes hashicorp/terraform-provider-google#11801

I am unable to get the newly added test to get working it throws error
provider_test.go:301: Step 1/2 error: After applying this test step, the plan was not empty.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: added internal IPv6 support on `google_compute_network` and `google_compute_subnetwork`

Derived from GoogleCloudPlatform/magic-modules#6105