Make ecs-anywhere-install.sh IPv6-only compatible

[amogh09]

Summary

This change makes ecs-anywhere-install.sh script compatible with IPv6-only environments. It now auto-detects if the instance is IPv6-only by looking at default IPv4 and IPv6 routes. An override option --ip-compatibility ipv4|ipv6 is also provided so that users can turn off auto-detection if it's not working as expected.

For IPv6-only environments, the script -

1. Creates an amazon-ssm-agent configuration file at /etc/amazon/ssm/amazon-ssm-agent.json with contents like below. These are dualstack endpoints of key SSM Agent dependencies as the current version of SSM Agent is not compatible with IPv6-only environments without such manual configuration.

{
        "Ssm": {
                "Endpoint": "https://ssm.ap-northeast-2.api.aws"
        },
        "Mgs": {
                "Endpoint": "https://ssmmessages.ap-northeast-2.api.aws"
        },
        "Mds": {
                "Endpoint": "https://ec2messages.ap-northeast-2.api.aws"
        }
}

This is a temporary workaround until we have an SSM Agent version that is IPv6-only compatible out of the box.

2. Makes the S3 URLs used for downloading various files us dualstack endpoints which are compatible with IPv6-only environments.

Testing

Tested on CentOS 9 and Amazon Linux 2023 running in IPv6-only networks. The script correctly auto-detected IPv6-only environment on these instances and successfully registered the instance with ECS. Also tested that the new --ip-compatibility command line argument works as expected and overrides auto-detection.

New tests cover the changes: NA

Description for the changelog

Enhancement: Make ecs-anywhere-install.sh compatible with IPv6-only environments

Additional Information

Does this PR include breaking model changes? If so, Have you added transformation functions?

Does this PR include the addition of new environment variables in the README?

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ShelbyZ]

Is there a case where a customer would create this file and prevent the updating to ipv6 corrected endpoints?

[amogh09]

Not known but my thought is that the script should honor pre-existing configuration and assume it to work correctly.