create resources to be accessible only behind vpn

>**Note**: If your feature-request is regarding the AWS Amplify Console service, please log it in the
[AWS Amplify Console repository](https://github.com/aws-amplify/amplify-console/issues)

**Is your feature request related to a problem? Please describe.**
I am working on a project that requires my API and other resources only be available from a corporate network (VPN). It is my understanding that resources created with the Amplify CLI do not support this. I'm sure there could be a workaround to edit the created network stacks to make this work, but I would be worried that any changes would get overridden when the generated Cloud Formation stacks are redeployed.

**Describe the solution you'd like**
The ability for Amplify CLI to generate resources with network stacks that support private only (VPN) access.