Allow clients to add addons to existing services

Request

So far it is only possible to add new addons to existing services via the admin panel. Please implement this feature for the customer panel as well. Customers should be able to purchase addons for existing services. This function should be able to allow and disallow for each individual addon. For example: a customer has purchased a web hosting package with 5 GB of storage. To get more storage, the customer could purchase additional storage via addon without upgrading to a new package.