add new timeout header

We should add the new `x-timeout-ms` header on get_header, to better avoid timeouts if the relay plays timing games.
Mev boost: https://github.com/flashbots/mev-boost/pull/817
Buiilder spec: https://github.com/ethereum/builder-specs/issues/130