Feature Request: Post-processor to upload VHD to Azure

_This issue was originally opened by @ptveite as hashicorp/packer#5477. 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._

I'm using the Hyper-V iso builder to make a base image for use with Azure BYOL, and would like to have a post-processor that will convert my vhdx output to vhd and upload it to an Azure storage container.

Start by locating the Azure plugin's post-processor entry points and its existing image or storage integration. Define the conversion and upload flow for Hyper-V VHDX output, then verify that a converted VHD reaches the requested Azure storage container.

Written by the indexing model from the issue text.