Append additional bytes to block and transaction byte array.

[MarcoOl94]

It's allow to add additional bytes at the end of the byte array representing a block or a transaction (SidechainBlock.serialize(), SidechainCoreTransactionSerializer.serialize()).
The modified data is sent to the network with these additional bytes and it's validated by the other nodes by just discarding these bytes (they are ignored in the deserialization process).