**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.
**What**
Add `POST /admin/regions`
Add `POST /admin/regions/:id`
Add `DELETE /admin/regions/:id`
All are added for v2 using API Routes and workflows.
In follow-up PRs, I will add support for passing countries in update and create.
Update: First follow-up PR is #6372
