Fix ApplicableTradeTax.tax_point_date field type

[martaprilmay]

Fix: Use DateField for tax_point_date instead of DateTimeField

Problem:

The tax_point_date field in ApplicableTradeTax was using DateTimeField. According to the EN16931, XRechnung XSD schemas and docs TaxPointDate should be of type udt:DateType.

Solution:

Created a new DateField class that properly handles date-only fields by generating and parsing <udt:DateString> elements, following the same pattern as the existing DateTimeField implementation.