feat: add age validation for admin contacts

- Add AgeValidation module for consistent age checks
- Validate admin contacts must be at least 18 years old
- Move age validation logic from Domain to shared module
- Add tests for admin contact age validation
- Fix JSON format for admin_contacts_allowed_ident_type setting

This change ensures that administrative contacts must be adults (18+),
using the same age validation logic as for registrants. The validation
works with both birthday and Estonian ID formats. Settings are now
properly stored as JSON strings for consistent parsing.

test/models/domain_test.rb

@@ -587,7 +587,14 @@ class DomainTest < ActiveSupport::TestCase
     assert domain.invalid?
     assert_includes domain.errors.full_messages, 'Admin domain contacts Admin contacts count must be between 1-10'
     
-    domain.admin_domain_contacts.build(contact: contacts(:john))
+    admin_contact = contacts(:john)
+    admin_contact.update!(
+      ident_type: 'priv',
+      ident: '37810166020',
+      ident_country_code: 'EE'
+    )
+    
+    domain.admin_domain_contacts.build(contact: admin_contact)
     assert domain.valid?
   end