Azure builder should enable "boot diagnostics" in the building VM

_This issue was originally opened by @jasonzio as hashicorp/packer#7531. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

The Azure builder can timeout with "Waiting for SSH to become available", but there's no way to investigate what caused the timeout. By enabling "boot diagnostics" in the building VM, at the very lease one could "Connect to serial console" to see if the log spew explained the problem.

Start by locating the Azure builder's building-VM creation path and the point where it waits for SSH. Check how boot diagnostics and serial-console access are configured for Azure VMs. Done means boot diagnostics is enabled for the building VM so SSH timeouts can be investigated through the serial console.

Written by the indexing model from the issue text.