getZxy with Azure Function app entry point

[tengl]

This PR adds code that can be used to deploy protomaps on Azure. See README.md for more info.

It is intended to be deployed in an Azure Function App what is exposed though a CDN.

The pmtiles file can be stored at any URL, but if you use Azure then you probably want to deploy it to Azure Blob Storage. Just make sure the function app has access to the container/file.

This exact code has note been tested. I just used it as base for a Function App, which has some modifications that I didn't want to include here.

Most of the code should work for any Source and any back-end, with minor modifications.