Add `SoapOrderPaymentService` and refactor current classes

This PR adds the SoapOrderPaymentService, along with a few refactorings:

1. Constants that were defined in OrderHeader in !41 (merged) are now moved to edu\wisc\services\cbs\common\MagentoOrder. They are common across multiple classes (OrderHeader and OrderPayment for sure) so now they're defined in one place.
2. OrderPaymentServiceResponse no longer has a paymentLineIdRef field as it isn't returned with the generated OutputParameters.
3. Setters in OrderPayment return $this like the others.

paidToDate in OrderPaymentServiceResponse doesn't have a defined type yet so I've replaced occurrences with DateTime, since I'm guessing that's what it should be.

Please review: @ahoffmann @weizhong-wang @lloyd-carter