Add callback method to retrieve the generated raw SBD

[RovoMe]

On using the Phase4PeppolSender.Builder and passing the bytes of the UBL via the builder's setPayload(byte[]) method, there is currently no way to obtain the generated SBD. While technically the SBD could be obtained via the output stream that is fed into our customized IAS4OutgoingDumper implementation, further filtering and decryption are necessary. Especially the decryption part is probably the blocking part though as we do not have access to the targets private key unfortunately.

A callback method that is invoked right after the SBD was generated would therefore be highly appreciated. The callback should probably support deep copying of the SBD in order to prevent posterior changes done to the original SBD while handling the callback.

[phax]

Part of 0.9.13 release