[WebApp] Incorrect Swagger for MSDeploy definition

In this Swagger: https://github.com/Azure/azure-rest-api-specs/blob/current/specification/web/resource-manager/Microsoft.Web/2016-08-01/WebApps.json#L15939

The addOnPackages property is ignored according to https://github.com/Azure/autorest/issues/2762, which is probably not the intended behavior.

The correct behavior should list `MSDeployCore` as a composition - like this:
```patch
diff --git a/specification/web/resource-manager/Microsoft.Web/2016-08-01/WebApps.json b/specification/web/resource-manager/Microsoft.Web/2016-08-01/WebApps.json
index 80858fa..efc3673 100644
--- a/specification/web/resource-manager/Microsoft.Web/2016-08-01/WebApps.json
+++ b/specification/web/resource-manager/Microsoft.Web/2016-08-01/WebApps.json
@@ -15936,8 +15936,12 @@
],
"properties": {
"properties": {
- "$ref": "#/definitions/MSDeployCore",
"description": "Core resource properties",
+ "allOf": [
+ {
+ "$ref": "#/definitions/MSDeployCore"
+ }
+ ],
"properties": {
"addOnPackages": {
"description": "List of Add-On packages. Add-On packages implicitly enable the Do Not Delete MSDeploy rule.",
```