Add support for Functions programming model v4

[ejizba]

Here's a blog post about the v4 model, if you want more info: https://techcommunity.microsoft.com/t5/apps-on-azure-blog/azure-functions-version-4-of-the-node-js-programming-model-is-in/ba-p/3773541

It's currently in preview, but we plan to GA pretty soon. The v4 model cleaned up some of the invocation context and request objects, which is the pretty much the only reason the applicationinsights package has to do anything.

Fixes https://github.com/Azure/azure-functions-nodejs-library/issues/110

My testing

I have a v4 http trigger making this get request inside the function:

await axios.get('https://functionscdn.azureedge.net/public/cli-feed-v4.json');

And it's showing up under the overarching http trigger request in app insights, so I think that means it's working.

[ejizba]

cc @hossam-nasr @hectorhdzg

[ejizba]

ping @hectorhdzg

[hectorhdzg]

@ejizba thanks for updating this, I will take a look soon, I had been kind of busy last couple of days.