Standardize state migration script

[morgante]

Currently, we have state migration scripts scattered across many different repos with a new script being created for each migration. For example:

• https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/blob/master/helpers/migrate7.py
• https://github.com/terraform-google-modules/terraform-google-network/blob/master/helpers/migrate.py
• https://github.com/terraform-google-modules/terraform-google-project-factory/blob/master/helpers/migrate.py

We should create a consolidated script (potentially integrated into the CFT CLI) which can be used for all migrations, with individual migrations simply providing a JSON migration config.

[github-actions[bot]]

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