Adding publisher field to VMExtension model (Azure#8398)

Author: hannah-murphy-0

specification/azsadmin/resource-manager/compute/Microsoft.Compute.Admin/preview/2015-12-01-preview/VMExtensions.json

@@ -224,6 +224,10 @@
           "description": "Target virtual machine operating system type necessary for deploying the extension handler.",
           "$ref": "PlatformImages.json#/definitions/OsType"
         },
+        "publisher": {
+          "description": "The publisher of the VM Extension",
+          "type": "string"
+        },
         "computeRole": {
           "description": "Compute role",
           "type": "string"