Azure Disk created with Public Endpoint, no option to make it private

_This issue was originally opened by @NickLarsenNZ as hashicorp/packer#10535. 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._

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#### Description

Using the `azure-arm` builder, a Disk is created with "Public Endpoint" enabled, even when building on a private network.

It would be great to have a config option to make it private (and make that the default).

#### Use Case(s)

Organisations with controls to prevent public disks being created.

#### Potential configuration

Either add an option to allow the disk to be public, but ***default to private***:

```json
"builders": [
{
"type": "azure-arm",
...
"pubic_disk_access": true
}
]
```

Or, hard code it to private (I can't think of a case where it would need to be public).

Maybe even the "Deny All" option would work, I'm not quite sure if private access is even required for mounting the disk to a VM.

#### Potential References

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