fix(medusa): calculates correct taxes and totals on order with gift cards (#1807)

**What**
Since the release of the Tax API the line item totals calculations on orders with gift cards have been wrong. To understand the bug consider the below order:

Region:
- tax_rate: 25%
- gift_cards_taxable: true

Order:
- applied gift card: 1000
- items: 
  - A: unit_price: 1000
  - B: unit_price: 500
- Subtotal: 1500

**Previous calculation method**
1. Determine how much of the gift card is used for each item using `item_total / subtotal * gift_card_amount`:
  - Item A: 1000/1500 * 1000 = 666.67
  - Item B: 500/1500 * 1000 = 333.33
2. Calculate line item totals including taxes using `(unit_price - gift_card) * (1 + tax_rate)`
  - Item A: 1000 - 666.67 = 333.33; vat amount -> 83.33
  - Item B: 500 - 333.33 = 166.67; vat amount -> 41.67
3. Add up the line item totals: order subtotal = 500; vat amount = 125; total = 625

This is all correct at the totals level; but at the line item level we should still use the "original prices" i.e. the line item total for item a should be (1000 * 1.25) = 1250 with a tax amount of 250. 

**New calculation method**
1. Use default totals calculations
  - Item A: subtotal: 1000, tax_total: 250, total: 1250
  - Item B: subtotal: 500, tax_total: 125, total: 625
2. Add up the line item totals: subtotal: 1500, tax_total: 375, total: 1875
3. Reduce total with gift card: subtotal: 1500 - 1000 = 500, tax_total: 375 - 250 = 125, total = 625

Totals can now be forwarded correctly to accounting plugins.

Fixes CORE-310.

packages/medusa/src/models/cart.ts

@@ -252,6 +252,7 @@ export class Cart extends SoftDeletableEntity {
   subtotal?: number
   refundable_amount?: number
   gift_card_total?: number
+  gift_card_tax_total?: number
 
   @AfterLoad()
   private afterLoad(): void {

packages/medusa/src/models/gift-card-transaction.ts

@@ -42,6 +42,12 @@ export class GiftCardTransaction {
   @CreateDateColumn({ type: resolveDbType("timestamptz") })
   created_at: Date
 
+  @Column({ nullable: true })
+  is_taxable: boolean
+
+  @Column({ type: "real", nullable: true })
+  tax_rate: number | null
+
   @BeforeInsert()
   private beforeInsert(): void {
     this.id = generateEntityId(this.id, "gct")

packages/medusa/src/models/order.ts

@@ -249,6 +249,7 @@ export class Order extends BaseEntity {
   paid_total: number
   refundable_amount: number
   gift_card_total: number
+  gift_card_tax_total: number
 
   @BeforeInsert()
   private async beforeInsert(): Promise<void> {