Adding middleware to new programming model

[aaronpowell]

This builds on some of the ideas proposed in Azure/azure-functions-nodejs-worker#664

Here's an idea I have for how we could do middleware:

import { app } from "@azure/functions";

app.get("home", () => ({ body: "Hello World" });

app.middleware({
  hookName: "preInvocation",
  hook: (context) => {
    //do stuff
  }
});

app.preInvocation(context => {
  //do stuff
});

app.middleware({
  hookName: "preInvocation",
  filter: "httpTrigger"
  hook: () => {}
});

app.middleware({
  hookName: "preInvocation",
  filter: (context) => context.trigger.url.contains("foo"),
  hook: () => {}
});

There's probably a bunch of additional permutations that we could support to make it more akin to the kind of middleware like you would come across in web frameworks, but also support the scenarios of having different trigger types within your application.

[aaronpowell]

This is a duplicate of #7 (I really should have searched issues first 😅)