feat(tax): add tax provider support (#6492)

**What**
- Adds Tax Provider model
- Adds loader to get Tax Provider from module options
- Adds System Tax provider which forwards tax rates as is

packages/types/src/tax/common.ts

@@ -33,7 +33,7 @@ export interface TaxRateDTO {
   /**
    * The ID of the user that created the Tax Rate.
    */
-  created_by: string
+  created_by: string | null
 }
 
 export interface FilterableTaxRateProps
@@ -138,7 +138,7 @@ export interface TaxCalculationContext {
 }
 
 interface TaxLineDTO {
-  rate_id: string
+  rate_id?: string
   rate: number | null
   code: string | null
   name: string

packages/types/src/tax/index.ts

@@ -1,3 +1,4 @@
 export * from "./common"
 export * from "./mutations"
 export * from "./service"
+export * from "./provider"

packages/types/src/tax/provider.ts

@@ -0,0 +1,48 @@
+import {
+  ItemTaxLineDTO,
+  ShippingTaxLineDTO,
+  TaxCalculationContext,
+  TaxRateDTO,
+  TaxableItemDTO,
+  TaxableShippingDTO,
+} from "./common"
+
+/**
+ * A shipping method and the tax rates configured to apply to the
+ * shipping method.
+ */
+export type ShippingTaxCalculationLine = {
+  /**
+   * The shipping method to calculate taxes for.
+   */
+  shipping_line: TaxableShippingDTO
+  /**
+   * The rates applicable on the shipping method.
+   */
+  rates: TaxRateDTO[]
+}
+
+/**
+ * A line item and the tax rates configured to apply to the
+ * product contained in the line item.
+ */
+export type ItemTaxCalculationLine = {
+  /**
+   * The line item to calculate taxes for.
+   */
+  line_item: TaxableItemDTO
+  /**
+   * The rates applicable on the item.
+   */
+  rates: TaxRateDTO[]
+}
+
+export interface ITaxProvider {
+  getIdentifier(): string
+
+  getTaxLines(
+    itemLines: ItemTaxCalculationLine[],
+    shippingLines: ShippingTaxCalculationLine[],
+    context: TaxCalculationContext
+  ): Promise<(ItemTaxLineDTO | ShippingTaxLineDTO)[]>
+}

packages/types/src/tax/service.ts

@@ -95,7 +95,7 @@ export interface ITaxModuleService extends IModuleService {
   ): Promise<TaxRateRuleDTO[]>
 
   getTaxLines(
-    item: (TaxableItemDTO | TaxableShippingDTO)[],
+    items: (TaxableItemDTO | TaxableShippingDTO)[],
     calculationContext: TaxCalculationContext,
     sharedContext?: Context
   ): Promise<(ItemTaxLineDTO | ShippingTaxLineDTO)[]>