provide access to validator set to track proposals correctly

[ralexstokes]

mev-boost-rs currently lacks access to the validator set so when it gets a public key in a BidRequest, it cannot match it to the corresponding block during open_bid.

the RelayMux should be given at least a mapping from pub key to validator index to facilitate the proposal pipeline

[ralexstokes]

I believe flashbots/mev-boost just keeps the most recent public key from the getHeader call and assumes this is the one that matches the getPayload call.

We should consider if there is any possible attack scenario or security consideration to doing this. If not, we should maybe even just drop the validation.

If we determine we should track this, then we can use the ValidatorRegistry component from the mev-rs crate as a component to mev-boost -- note that this requires a live beacon API to function.