Feature Request - Azure (ARM) Postprocessor

_This issue was originally opened by @flamarion as hashicorp/packer#6281. 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._

Hi,

I am a daily user of Packer and I'd like to ask you guys to try implement some post-processors for Azure like we have for AWS.

Currently, after all my builds (virtualbox-iso, for example) which I need to push the result to Azure, I need to do it using a post-processor shell and wrap Python + Virtualenv + AzureCLI.

To use these tools is not a problem at all, but the post-processor is being really big and complex due the nature of build (We need to do some conversions and resizes to be accepted by Azure) so the part to push the result of all those complexity could be facilitated with a post-processor able to create a managed image and VHD.

Thank you,

Flamarion Jorge