feat(Forms): add `isAdultValidator` to Field.NationalIdentityNumber

[tujoworker]

This is a draft PR with the intention to add isAdultValidator.

As of now, it looks to me that it works as expected. See the video how I tested it manually: https://github.com/user-attachments/assets/27cddbaf-457f-4915-849e-0bde111b1ba1

To dos:

• [ ] Have a discussion on how we should export the validator.
• [ ] Add tests.
• [ ] Add docs about it.