New: shipment addresses filter hook

[alexmigf]

This pull request introduces a new filter postnl_shipment_addresses to modify shipment addresses before generating PostNL shipping labels. The filter is necessary to handle specific address formats used in our plugin, which appends the house number directly to the address_1 field. Without this filter, the shipping label would display the house number twice—once in address_1 and once in the HouseNr field.

Usage:

add_filter( 'postnl_shipment_addresses', function( $addresses, $client ) {
	if ( ! empty( $addresses[0]['HouseNr'] ) ) {
		$addresses[0]['HouseNr'] = '';
	}
	
	return $addresses;
}, 10, 2 );

Benefits:

• Ensures correct address formatting for our plugin, preventing duplicated house numbers on PostNL shipping labels.
• Provides a flexible solution that could benefit other scenarios with different address formatting requirements.