Stateful MIG -> GA

[modular-magician]

Fixes hashicorp/terraform-provider-google#7323.

Stateful MIG is controlled with PerInstanceConfig and RegionPerInstanceConfig. This PR makes those both available in GA as well as related utilities and tests.

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: Marked `stateful_disk` as GA in `google_compute_ instance_group_manager` 

compute: Marked `stateful_disk` as GA in `google_compute_ region_instance_group_manager` 

compute: Marked `google_compute_ per_instance_config` as GA

compute: Marked `google_compute_ region_per_instance_config` as GA

Derived from GoogleCloudPlatform/magic-modules#4047