(refactor) L1: Centralize code encoding for P2P Messages

[fkrause98]

When sending a P2P message to another node, a simple but easily missable step is that besides encoding the data for the message itself,
you need to also encode the code assigned to the message. For example, the P2P message "getAccountRange", has code 0x21, which needs to be encoded alongside the data it sends. Currently, this code encoding is done a bit 'manually', like in this example.
It would be better to have a centralized way (a function plus a match, maybe) to add said codes when encoding the messages, or maybe add it to the RLPxEncode trait itself.