Make ValidationRules a regular object, add country_code and split postal_code_examples

ValidationRules is no longer a namedtuple as indexing it using integer keys did not make a lot of sense. This will allow us to add new fields without breaking compatibility in future.

The ValidationRules instance now has a country_code field along with the previously available country_name.

⚠️ The ValidationRules.postal_code_examples is now a list instead of a string. This is a backwards-incompatible change and if your code relies on the examples it needs to be adapted.