relay: finish signature verification

[ralexstokes]

the relay wants to verify the signed blinded beacon block it gets from proposers before releasing the execution payload

the current implementation is unfinished, with a "todo" here: https://github.com/ralexstokes/mev-rs/blob/main/mev-build-rs/src/signing.rs#L18

the thing we need to do is pass in the correct values so that verify_signed_consensus_message can compute the correct domain

as named, this function suggests it can verify any consensus message and so it should:

1. not hard-code the DomainType -- which suggests this should be a parameter the caller provides
2. supply the correct fork_version -- this will vary with each message and the caller can look at the slot against the context to get the right value
3. genesis_validators_root should be fine but worth double-checking the implementation

addressing this issue means addressing each of these points and then ideally also adding some tests of recent blocks from each network, mainnet, sepolia, and goerli

[ralexstokes]

@PatStiles is going to work on this :)

[ralexstokes]

this issue has become a little stale, with most of the above being taken care of in #70

that being said, there is one small TODO left to wrap up the original items from the above post:

https://github.com/ralexstokes/mev-rs/blob/main/mev-relay-rs/src/relay.rs#L78

which should get the correct genesis_validators_root instead of just using a Default value