Deploy a function app with an Event Grid subscription in a single ARM template

It needs to be possible to create a V2 function app, along with an Event Grid subscription that is configured to deliver events to the function app, all within a single ARM template. Today this is difficult to due limitations with the functions ARM keys API (discussed [here](https://github.com/Azure/azure-functions-host/wiki/Changes-to-Key-Management-in-Functions-V2#arm-impact)).