p2p/discover/v4wire: use optional RLP field for EIP-868 seq (#3963)

This changes the definitions of Ping and Pong, adding an optional field
for the sequence number. This field was previously encoded/decoded using
the "tail" struct tag, but using "optional" is much nicer.

see https://github.com/ethereum/go-ethereum/pull/22842



Co-authored-by: Felix Lange <fjl@twurst.com>