Support flag to override listening address on serve

[KenFigueiredo]

Hey there!

I'm attempting to run the credential-helper in a container within Kubernetes where it is a standalone pod in serve mode.
Currently this runs into issues where I'm unable to reach the application running in serve mode due to the listening address binding being hardcoded to 127.0.0.1.

I've ran some manual validation where I've changed: https://github.com/aws/rolesanywhere-credential-helper/blob/main/aws_signing_helper/serve.go#L23 to 0.0.0.0 and this seems to resolve the issue.

To support this use case it would be worthwhile to add an additional startup flag --listening-addr that overrides that value but defaults to localhost to keep the current behavior.

Thanks!

[dwmw2]

This should not be limited to Legacy IP only. It's over a quarter of a century since the IPv6 standard was published.