feat: proper determination of agency initiated bookings

For booking that are agency initiated (groups) different rules apply
concerning mutability and requirement of personal data and whether the
first participant is the applicant or not. This commit adds logic to
evaluate the agency id assigned to the booking data to decide.

src/BusProNet/XmlLoader/AgencyLoader.php

@@ -15,6 +15,7 @@ use Symfony\Contracts\Cache\ItemInterface;
 class AgencyLoader
 {
     public const DEFAULT_AGENCY_CODE = 'INTERN';
+    public const INTERNAL_AGENCY_CODE = '0004';
 
     public function __construct(
         private readonly CacheInterface $cache,