Feature Request: Azure Shared Image Gallery - Image definition created as part of builder

_This issue was originally opened by @Marcus-James-Adams as hashicorp/packer#8803. 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._

Following on from https://github.com/hashicorp/packer/issues/7809

Packer should be self-contained and be able to do everything required within a builder without outside assistance.

currently, we have to do the following before running packer

> Use az cli or PowerShell to create a shared image definition, e.g. for a customised Ubuntu 18.04-LTS image: az sig image-definition create --resource-group --gallery-name --gallery-image-definition --publisher --offer UbuntuServer --sku 18.04-LTS --os-type linux

It would be good if packer could create the definition automatically as part of the builder process.