Add support for InitialGroupConfig to google_cloud_identity_group

[modular-magician]

Fixes hashicorp/terraform-provider-google#7986.

If you change the value of initial_group_config after creation, as this PR is written, the group will be destroyed and recreated. Let me know if you'd like me to look at alternative behaviors here (e.g., being a no-op).

Thanks!

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).

  ▶️ I had trouble figuring out if the acceptance tests support selecting a billing project to use and our service accounts don't have permissions to create groups. However, I tested this change by hand in a separate module and it works.

• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

cloud_identity: add support for `initial_group_config` to the google_cloud_identity_group resource

Derived from GoogleCloudPlatform/magic-modules#4746