Update Azure "cloud_environment" option to allow user to directly set the metadata_url

_This issue was originally opened by @SwampDragons in https://github.com/hashicorp/packer/issues/10898 and has been migrated to this repository. The original issue description is below._

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This would allow users of custom azure environments to properly retrieve all of the necessary endpoints for their Azure build. Packer's current solution https://www.packer.io/docs/builders/azure/arm#cloud_environment_name is inflexible and doesn't grow to fit new custom environments.

Terraform has implemented this here: https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs#metadata_host

We can use their implementation as a guide.