Feat: replace checkout addon billing address component for jh

Override the billing address component when the instance is running as JH

ee/app/assets/javascripts/subscriptions/buy_addons_shared/components/checkout.vue

 <script>
 import updateState from 'ee/subscriptions/graphql/mutations/update_state.mutation.graphql';
-import BillingAddress from 'ee/vue_shared/purchase_flow/components/checkout/billing_address.vue';
+import BillingAddress from 'jh_else_ee/vue_shared/purchase_flow/components/checkout/billing_address.vue';
 import ConfirmOrder from 'ee/vue_shared/purchase_flow/components/checkout/confirm_order.vue';
 import PaymentMethod from 'ee/vue_shared/purchase_flow/components/checkout/payment_method.vue';
 import { GENERAL_ERROR_MESSAGE } from 'ee/vue_shared/purchase_flow/constants';