AWS ECR PUBLIC support

[matiri132]

The goal is add to the AWS module the capability to work with AWS ECR public repositories.
The AWS sdk contains a different module to get aws ECR public repositories.

This could be acomplished creating a new module (and test) in the actual Terratest AWS module based in the ecr module and modifying the dependency to "github.com/aws/aws-sdk-go/service/ecrpublic" instead of "github.com/aws/aws-sdk-go/service/ecr".