Fix bug in the aws_sns_topic_subscription.datadog_security resource

[marwinbaumannsbp]

• Resolves issue where leaving the sns_security_subscription variable empty would cause failure.
• Renamed and restructured the sns_security_subscription variable into sns_aws_config_subscription. Changed into a map to prevent the endpoint being used in the for_each. The endpoint can contain a secret (e.g. the datadog api key) and this would cause failure. In TF V0.14 you can't use for_each with an object that has sensitive values.
• Moved this resource out of the security_hub module since the SNS topic aws-controltower-AggregateSecurityNotifications only contains AWS Config data: https://docs.aws.amazon.com/controltower/latest/userguide/compliance.html