Support warmup trigger

[ejizba]

https://learn.microsoft.com/en-us/azure/azure-functions/functions-bindings-warmup?tabs=isolated-process&pivots=programming-language-javascript

Today you can use a warmup trigger with generics, but would be nice if people could do it using app.warmup.

I'm honestly not that familiar with the warmup trigger - I'm mostly filing this issue because we do see it used by Node.js customers in our telemetry. Further investigation is necessary