feat(tax): adds getItemTaxLines (#6440)

**What**
- Selects the correct tax line for an item given a calculation context.

**For later PR**
- Consider optimizations. Some thoughts:
  - Even with global sales the number of rates in the DB is not likely to grow beyond ~1000.
  - Can large orders with hundreds of items optimize somehow?
  - Does it make sense to write a custom SQL query to do this?
- Support combined rate.

**Test cases covered**
The selection of tax rates take the following priority:

1. specific product rules - province
2. specific product type rules - province
3. default province rules
4. specific product rules - country
5. specific product type rules - country
6. default country rules

There are test cases for each of them under the following data seed structure:

### **US** 
- **Default Rate**: 2%
- **Sub-Regions**
  - CA 
    - Default Rate: 5%
    - Overrides
      - Reduced rate (for 3 product ids): 3%
      - Reduced rate (for product type): 1%
  - NY 
    - Default rate: 6%
  - FL 
    - Default rate: 4%
- **Overrides**
  - None

### **Denmark** 
- **Default rate:** 25% 
- **Sub-Regions**
  - None
- **Overrides**
  -  None

### **Germany** 
- **Default Rate:** 19%
- **Sub-Regions**
  - None
- **Overrides:**
  - Reduced Rate (for product type) - 7%

### **Canada** 
- **Default rate**: 5%
- **Sub-Regions**
  - QC 
    - Default rate: 2%
    - Overrides:
      - Reduced rate (for same product type as country reduced rate): 1%
  - BC 
    - Default rate: 2%
- **Overrides**
  - Reduced rate (for product id) - 3%
  - Reduced rate (for product type) - 3.5%

packages/types/src/tax/common.ts

@@ -99,3 +99,55 @@ export interface FilterableTaxRateRuleProps
   updated_at?: OperatorMap<string>
   created_by?: string | string[] | OperatorMap<string>
 }
+export interface TaxableItemDTO {
+  id: string
+  product_id: string
+  product_name?: string
+  product_category_id?: string
+  product_categories?: string[]
+  product_sku?: string
+  product_type?: string
+  product_type_id?: string
+  quantity?: number
+  unit_price?: number
+  currency_code?: string
+}
+
+export interface TaxableShippingDTO {
+  id: string
+  shipping_option_id: string
+  unit_price?: number
+  currency_code?: string
+}
+
+export interface TaxCalculationContext {
+  address: {
+    country_code: string
+    province_code?: string | null
+    address_1?: string
+    address_2?: string | null
+    city?: string
+    postal_code?: string
+  }
+  customer?: {
+    id: string
+    email: string
+    customer_groups: string[]
+  }
+  is_return?: boolean
+}
+
+interface TaxLineDTO {
+  rate_id: string
+  rate: number | null
+  code: string | null
+  name: string
+}
+
+export interface ItemTaxLineDTO extends TaxLineDTO {
+  line_item_id: string
+}
+
+export interface ShippingTaxLineDTO extends TaxLineDTO {
+  shipping_line_id: string
+}

packages/types/src/tax/service.ts

@@ -8,6 +8,11 @@ import {
   TaxRegionDTO,
   TaxRateRuleDTO,
   FilterableTaxRateRuleProps,
+  TaxableItemDTO,
+  TaxCalculationContext,
+  ItemTaxLineDTO,
+  ShippingTaxLineDTO,
+  TaxableShippingDTO,
 } from "./common"
 import {
   CreateTaxRateRuleDTO,
@@ -64,4 +69,10 @@ export interface ITaxModuleService extends IModuleService {
     config?: FindConfig<TaxRateRuleDTO>,
     sharedContext?: Context
   ): Promise<TaxRateRuleDTO[]>
+
+  getTaxLines(
+    item: (TaxableItemDTO | TaxableShippingDTO)[],
+    calculationContext: TaxCalculationContext,
+    sharedContext?: Context
+  ): Promise<(ItemTaxLineDTO | ShippingTaxLineDTO)[]>
 }