fix(in-app-purchase2): Registering a product with an alias is now opt…

…ional (#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;
 }