Add DNS check to consolidated readiness

[cliffano]

Currently we're relying on provisioning completeness done in SSM command pre step to verify whether the author-publish-dispatcher component has been provisioned and ready.

This should be improved further with adding DNS check, e.g. publish-dispatcher content can be accessed via the DNS for the stack-prefix being checked.

The implementation of this DNS check can also be reused later for full-set readiness check.