Enable Boot Diagnostics with Managed Storage Account

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#### Description

As recommended by [Microsoft](https://learn.microsoft.com/en-us/azure/virtual-machines/boot-diagnostics#boot-diagnostics-storage-account), using a managed storage account for boot diagnostics provides significant performance improvements when creating Azure VMs, by eliminating the need to create a separate storage account for boot diagnostics data.

Additionally, it simplifies management by removing any consideration for manually cleaning up storage accounts that store the serial console log and screenshot file after the VM build is completed.

#### Potential configuration

The ideal solution would be to implement it the same way it is done for a Virtual Machine in Terraform, using an optional block: https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/windows_virtual_machine#boot_diagnostics

```
boot_diagnostics {
storage_account_uri = null
}
```

NOTE: Passing a null value will utilize a Managed Storage Account to store Boot Diagnostics.

#### Potential References

I was hoping this could be as simple as implementing a conditional if statement to make the following property (`StorageUri`) optional: https://github.com/hashicorp/packer-plugin-azure/blob/93909bcb17b16d207ee60207ff50d7cf8809b47d/builder/azure/common/template/template_builder.go#L539

Start in builder/azure/common/template/template_builder.go around the StorageUri property, and compare the optional boot_diagnostics behavior in the Terraform Azure VM resource. Confirm how a null or omitted storage URI is passed to Azure, then verify that VM boot diagnostics use a managed storage account without requiring a separate account.

Written by the indexing model from the issue text.