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/core-flows/src/region/steps/update-regions.ts

@@ -48,6 +48,7 @@ export const updateRegionsStep = createStep(
         name: r.name,
         currency_code: r.currency_code,
         metadata: r.metadata,
+        countries: r.countries.map((c) => c.iso_2),
       }))
     )
   }