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

[mcgirr]

WIP for #312

• 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.

[mcgirr]

I have a project demonstrating this working that I can include snippets from for this PR