Add example for creating custom resources

[fwang]

Do not merge this example yet.

This shows how to create a CloudFormation custom resource using SST. Currently you can deploy (ie. sst deploy) this stack, but sst start would not work. That's because the custom resource Lambda function will be replaced by a stub, but the local websocket server is not running during the deployment.

Hence, we need to either

• start the websocket server before the deployment starts;
• or educate ppl not to use sst.Function when creating custom resources.