Optimise and refine SingleAttestation conversion (#6934)

Closes

- https://github.com/sigp/lighthouse/issues/6805


  - 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 `SingleAttestation`s 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.

beacon_node/beacon_chain/src/lib.rs

@@ -54,6 +54,7 @@ mod pre_finalization_cache;
 pub mod proposer_prep_service;
 pub mod schema_change;
 pub mod shuffling_cache;
+pub mod single_attestation;
 pub mod state_advance_timer;
 pub mod sync_committee_rewards;
 pub mod sync_committee_verification;