Refactor/stream vc vote publishing

[shane-moore]

Proposed Changes

Changes four ValidatorStore batch signing methods to return impl Stream instead of Future. Services consume the stream and publish each batch as it arrives. No behavioral change for lh since LighthouseValidatorStore wraps everything in stream::once

Also replaces anonymous tuples in method signatures with named structs

Motivation

Anchor needs to yield signing results per SSV committee. Each committee runs consensus independently, so streams let faster committees publish without waiting on slower ones.

[shane-moore]

@dknopik @michaelsproul, made some helper structs like ContributionToSign to replace inline tuple structs like

(
            u64,
            PublicKeyBytes,
            SyncCommitteeContribution<Self::E>,
            SyncSelectionProof,
        )

wdyt?

[shane-moore]

I was originally doing return Err('...') in this variant after the log. I think an issue with that is that if we hit an error with one batch, then we'd early return before the rest could be processed

also, propagating the error up just logs an error, which we're doing with the crit! anyways, so seems fine to allow the stream to complete and hit the Ok(), since the logging happens regardless

open to thoughts from @dknopik and otherwise