code: public AddOns getAddOns(final String planCode) {
          return doGET(Plan.PLANS_RESOURCE +
                  "/" +
                  planCode +
                  AddOn.ADDONS_RESOURCE,
                  AddOns.class,
                  new QueryParams());
      }
docs: Return all the {@link AddOn} for a {@link Plan}
  <p>
  
  @param planCode
  @return the {@link AddOn} objects as identified by the passed plan ID