feat(tax): normalize country and province code (#6513)

**What**
- Ensures that country and province codes are always stored as lower case.
- Ensures that calculation context normalizes input before getting region rates.

packages/tax/integration-tests/__tests__/index.spec.ts

@@ -34,14 +34,14 @@ moduleIntegrationTestRunner({
           expect.arrayContaining([
             expect.objectContaining({
               id: region.id,
-              country_code: "US",
+              country_code: "us",
               province_code: null,
               parent_id: null,
             }),
             expect.objectContaining({
               id: provinceRegion.id,
-              country_code: "US",
-              province_code: "CA",
+              country_code: "us",
+              province_code: "ca",
               parent_id: region.id,
             }),
           ])
@@ -397,6 +397,7 @@ moduleIntegrationTestRunner({
         expect(taxRegions).toEqual([
           expect.objectContaining({
             id: region.id,
+            country_code: "us",
             deleted_at: expect.any(Date),
           }),
         ])