add external ipv6 support

[modular-magician]

Fixes hashicorp/terraform-provider-google#9860

I'm unsure if these fields are updateable. The documentation seems to suggest it is (This field can be both set at instance creation and update network interface operations.) however, the documentation for the updateNetworkInterface seems to suggest This method can only update an interface's alias IP range and attached network. I tried a few things, but only ran into errors, so left it off.

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 external IPv6 support on `google_compute_subnetwork` and `google_compute_instance.network_interfaces`

Derived from GoogleCloudPlatform/magic-modules#5241