feat(regions): Add support for updating countries in a region (#6432)

**What**
Add missing support for updating countries on the POST /admin/regions/:id route.
Furthermore, the PR refactors how the normalization and validation was handled in both create and update scenarios.
Finally, the PR adds a unique index on the country entity, which ensures at the DB level that we cannot have a duplicate country in the DB for a single region.

packages/types/src/region/mutations.ts

@@ -7,21 +7,15 @@ export interface CreateRegionDTO {
 
 export interface UpdateRegionDTO {
   id: string
+  name?: string
   currency_code?: string
   countries?: string[]
-  name?: string
   metadata?: Record<string, unknown>
 }
 
 export interface UpdatableRegionFields {
-  currency_code?: string
   name?: string
+  currency_code?: string
+  countries?: string[]
   metadata?: Record<string, unknown>
 }
-
-export interface AddCountryToRegionDTO {
-  region_id: string
-  country_id: string
-}
-
-export interface RemoveCountryFromRegionDTO extends AddCountryToRegionDTO {}