retry failed validator registrations

[ralexstokes]

right now mev-boost-rs will just drop any failures on validator registration.

it should keep the failed registrations and retry them with some kind of backoff in the event that a relay has transient issues processing them.

I don't think we need to keep any state here as validators should be periodically submitting their registrations and if the updates are frequent enough then we can likely just ignore any need to retry.

Some relevant issues that will impact this:

ethereum/builder-specs#8
ethereum/builder-specs#13

[ralexstokes]

I don't think we need to keep any state here as validators should be periodically submitting their registrations and if the updates are frequent enough then we can likely just ignore any need to retry.

in practice, I think we can just rely on how frequently validators are sending their registrations and don't need very sophisticated retry logic here