WIP: Add initial support for IPv6 to the vpc and subnets modules

[ ] Update the changelog
[ ] Make sure that modules and files are documented. This can be done inside the module and files.
[ ] Make sure that new modules directories contain a basic README.md file.
[ ] Make sure that the module is added to tests/main.tf
[ ] Make sure that the linting passes on CI.
[ ] Make sure that there is an up to date example for your code:
- For new modules this would entail example code for how to use the module or some explanation in the module readme.
- For new examples please provide a README explaining how to run the example. It's also ideal to provide a basic makefile to use the example as well.
[ ] Make sure that there is a manual CI trigger that can test the deployment.

fpco / terraform-aws-foundation