Add Specialized Field for Shared Image Gallery Destination

[JenGoldstrich]

Adds specialized option to Shared Image Gallery Destination block, the plugin has previously always called Generalize which tells Azure to set the build virtual machine state to Generalized. Azure does not allow capturing Specialized VMs into managed images, so I've limited this to direct publish to Shared Image Gallery/Compute Gallery

https://learn.microsoft.com/en-us/azure/virtual-machines/vm-specialized-image-version

Closes #262

Example HCL template:

source "azure-arm" "autogenerated_1" {
  communicator    = "winrm"
  image_offer     = "WindowsServer"
  image_publisher = "MicrosoftWindowsServer"
  image_sku       = "2012-R2-Datacenter"
  location        = "South Central US"
  os_type         = "Windows"
  use_azure_cli_auth   = "true"
  vm_size         = "Standard_DS2_v2"
  winrm_insecure  = "true"
  winrm_timeout   = "3m"
  winrm_use_ssl   = "true"
  winrm_username  = "packer"
  shared_image_gallery_destination  {
    gallery_name        = "jennaspecialized"
    image_name          = "jenna"
    image_version       = "1.1.1"
    resource_group      = "jenna"
    specialized = "true"
  }
}

build {
  sources = ["source.azure-arm.autogenerated_1"]
}

[nywilken]

This looks good. Nicely done. One suggestion and one question. But otherwise good. Approving so you can merge when ready.

[nywilken]

What do you think about calling this attribute specialized to match the Azure CLI flag?

The behavior is the same as it is written the only change is the name of the configuration attribute.

Suggested change

	SigDestinationSkipGeneralization bool `mapstructure:"skip_generalization"`
	SigDestinationSkipGeneralization bool `mapstructure:"specialized"`

[nywilken]

NIce 👍

[nywilken]

LGTM