feat(core-flows,medusa,types,utils): adds update cart API with promotions (#6514)

what:

- adds update cart API
  - workflow
  - promotions
  - sales channel
  - region
  - customer

packages/cart/src/types/cart.ts

@@ -14,11 +14,11 @@ export interface CreateCartDTO {
 
 export interface UpdateCartDTO {
   id: string
-  region_id?: string | null
+  region_id?: string
   customer_id?: string | null
   sales_channel_id?: string | null
   email?: string | null
-  currency_code?: string | null
+  currency_code?: string
   metadata?: Record<string, unknown> | null
 
   adjustments?: (CreateLineItemAdjustmentDTO | UpdateLineItemAdjustmentDTO)[]