Create a true "express.js" programming model package

[ejizba]

The idea is we would create a separate npm package (@azure/functions-express?) that lets you bring your existing express.js app and it "just works" on Azure Functions. I assume this model would focus just on http, and not other azure triggers

[diberry]

@ejizba If someone wanted to bring a different backend as an azure function, there are various docs in the wild about how to do it on v3. Conceptually, as a place to start, how would someone do this? I'm not looking for a separate package as much as a guideline to bring any type of backend into the new model.