feat(tax): introduce tax override data models (#6422)

**What**
- Adds Tax rules to allow overrides of tax rates for certain products, product types or shipping options.

**Punted to future PR**
- Currently, the creation methods only support bulk operations. A later PR will include support for singular operations, too.
- It should be possible to add products, types, and shipping options to a tax rate after creating it. Add, remove, update will come in a later PR.

packages/tax/src/models/index.ts

@@ -1,2 +1,3 @@
 export { default as TaxRate } from "./tax-rate"
 export { default as TaxRegion } from "./tax-region"
+export { default as TaxRateRule } from "./tax-rate-rule"

packages/tax/src/models/tax-rate-rule.ts

@@ -0,0 +1,55 @@
+import {
+  Cascade,
+  Entity,
+  ManyToOne,
+  PrimaryKey,
+  PrimaryKeyProp,
+  Property,
+} from "@mikro-orm/core"
+import TaxRate from "./tax-rate"
+
+const TABLE_NAME = "tax_rate_rule"
+
+const taxRateIdIndexName = "IDX_tax_rate_rule_tax_rate_id"
+
+@Entity({ tableName: TABLE_NAME })
+export default class TaxRateRule {
+  @PrimaryKey({ columnType: "text" })
+  tax_rate_id!: string
+
+  @PrimaryKey({ columnType: "text" })
+  reference_id!: string;
+
+  [PrimaryKeyProp]?: ["tax_rate_id", "reference_id"]
+
+  @Property({ columnType: "text" })
+  reference: string
+
+  @ManyToOne(() => TaxRate, {
+    fieldName: "tax_rate_id",
+    index: taxRateIdIndexName,
+    cascade: [Cascade.REMOVE, Cascade.PERSIST],
+  })
+  tax_rate: TaxRate
+
+  @Property({ columnType: "jsonb", nullable: true })
+  metadata: Record<string, unknown> | null = null
+
+  @Property({
+    onCreate: () => new Date(),
+    columnType: "timestamptz",
+    defaultRaw: "now()",
+  })
+  created_at: Date
+
+  @Property({
+    onCreate: () => new Date(),
+    onUpdate: () => new Date(),
+    columnType: "timestamptz",
+    defaultRaw: "now()",
+  })
+  updated_at: Date
+
+  @Property({ columnType: "text", nullable: true })
+  created_by: string | null = null
+}