rlp: add support for optional struct fields (#22832) (#3977)

This adds support for a new struct tag "optional". Using this tag, structs used
for RLP encoding/decoding can be extended in a backwards-compatible way,
by adding new fields at the end.

see geth commit https://github.com/ethereum/go-ethereum/commit/700df1442d714cb3c42a602c39c042ce88be463f



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