Allow appending/prepending graffiti to client version info

We should add some config that allows users to append/prepend custom graffiti alongside their client version info (defined here: https://github.com/ethereum/execution-apis/blob/main/src/engine/identification.md)

Further standardisation of the graffiti format is defined here: https://hackmd.io/@wmoBhF17RAOH2NZ5bNXJVg/BJX2c9gja