fix(in-app-purchase2): Registering a product with an alias is now optional (#2946)

According to the documentation, alias is optional
https://github.com/j3k0/cordova-plugin-purchase/blob/master/doc/api.md#storeregisterproduct

src/@ionic-native/plugins/in-app-purchase-2/index.ts

@@ -73,7 +73,7 @@ import { Injectable } from '@angular/core';
 
 export interface IAPProductOptions {
   id: string;
-  alias: string;
+  alias?: string;
   type: string;
 }