feat(medusa,core-flows): complete cart [part-1] (#7201)

what:

- adds a very basic complete cart endpoint that creates an order
- the complete cart workflow currently does the following:
  - create tax lines
  - create order

packages/modules/order/src/types/order-detail.ts

@@ -12,7 +12,7 @@ export interface PartialUpsertOrderItemDTO {
   return_dismissed_quantity?: BigNumberInput
   written_off_quantity?: BigNumberInput
 
-  metadata?: Record<string, unknown>
+  metadata?: Record<string, unknown> | null
 }
 
 export interface CreateOrderItemDTO extends PartialUpsertOrderItemDTO {

packages/modules/order/src/types/order.ts

@@ -8,7 +8,7 @@ export interface CreateOrderDTO {
   currency_code: string
   status?: OrderStatus
   no_notification?: boolean
-  metadata?: Record<string, unknown>
+  metadata?: Record<string, unknown> | null
 }
 
 export interface UpdateOrderDTO {
@@ -21,5 +21,5 @@ export interface UpdateOrderDTO {
   currency_code?: string
   status?: OrderStatus
   no_notification?: boolean
-  metadata?: Record<string, unknown>
+  metadata?: Record<string, unknown> | null
 }