Usage of AwsCustomResource to retrieve IoTEndpoint prevents users from deploying with --hotswap

[Lewenhaupt]

When we use AwsCustomResource to retrieve the IoTEndpoint users cannot use cdk deploy --hotswap as Custom::AWS is not supported. The benefit of the custom resource is that we get it during deploy so we can add it to output and as env var for all lambdas. An alternative would be to actually look it up during runtime for both the client side listener and the lambdas. A different alternative could be to try if we can push the ServerlessSpy stuff to it's own nested stack. Perhaps that could help us isolate it better and allow --hostwap to work as long as no changes have been made to the ServerlessSpy nested stack?