Optimise and refine SingleAttestation conversion

[michaelsproul]

Issue Addressed

Closes

• Ensure SingleAttestation conversion happens on the BeaconProcessor #6805

Proposed Changes

• Use a new WorkEvent::GossipAttestationToConvert to handle the conversion from SingleAttestation to Attestation on the beacon processor (prevents a Tokio thread being blocked).
• Improve the error handling for single attestations. I think previously we had no ability to reprocess single attestations for unknown blocks -- we would just error. This seemed to be the case in both gossip processing and processing of SingleAttestations from the HTTP API.
• Move the SingleAttestation -> Attestation conversion function into beacon_chain so that it can return the attestation_verification::Error type, which has well-defined error handling and peer penalties. The now-unused variants of types::Attestation::Error have been removed.

Additional Info

The names of variables and variants are quasi-generic so that we can switch around the conversion direction, i.e. Attestation<E> -> SingleAttestation, and run the whole attestation pipeline on SingleAttestation only. This will be more efficient for SingleAttestations as we can postpone the expensive committee lookup until after signature verification (which is the whole point of SingleAttestation). I made a start on this here, but it needs more work:

• Fuller implementation of SingleAttestation #6923

See also:

• SingleAttestation full implementation #6616

[eserilev]

LGTM!

Would be nice to get a second set of eyes on the review though

[pawanjay176]

Nice and tidy! Tested this on devnet 6 and mainnet, logs and metrics looks good