Use newer AWS integration CFN templates
Since I last updated this construct, Datadog now uses new CFN templates. The main "quick-start" template is here: https://datadog-cloudformation-template-quickstart.s3.amazonaws.com/aws/main_v2.yaml
I don't think we'll want to use the App Key and, instead, have the user pass their external ID directly.
The main issue is that the https://datadog-cloudformation-template-quickstart.s3.amazonaws.com/aws/datadog_integration_api_call_v2.yaml template does not accept ApiKey / AppKey as a secret. We'd have to work around this.
I filed an upstream issue with Datadog to see if they'll support passing the sensitive parameters as Secret ARNs: https://github.com/DataDog/cloudformation-template/issues/68