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/single_attestation.rs

@@ -0,0 +1,46 @@
+use crate::attestation_verification::Error;
+use types::{Attestation, AttestationElectra, BitList, BitVector, EthSpec, SingleAttestation};
+
+pub fn single_attestation_to_attestation<E: EthSpec>(
+    single_attestation: &SingleAttestation,
+    committee: &[usize],
+) -> Result<Attestation<E>, Error> {
+    let attester_index = single_attestation.attester_index;
+    let committee_index = single_attestation.committee_index;
+    let slot = single_attestation.data.slot;
+
+    let aggregation_bit = committee
+        .iter()
+        .enumerate()
+        .find_map(|(i, &validator_index)| {
+            if attester_index as usize == validator_index {
+                return Some(i);
+            }
+            None
+        })
+        .ok_or(Error::AttesterNotInCommittee {
+            attester_index,
+            committee_index,
+            slot,
+        })?;
+
+    let mut committee_bits: BitVector<E::MaxCommitteesPerSlot> = BitVector::default();
+    committee_bits
+        .set(committee_index as usize, true)
+        .map_err(|e| Error::Invalid(e.into()))?;
+
+    let mut aggregation_bits =
+        BitList::with_capacity(committee.len()).map_err(|e| Error::Invalid(e.into()))?;
+    aggregation_bits
+        .set(aggregation_bit, true)
+        .map_err(|e| Error::Invalid(e.into()))?;
+
+    // TODO(electra): consider eventually allowing conversion to non-Electra attestations as well
+    // to maintain invertability (`Attestation` -> `SingleAttestation` -> `Attestation`).
+    Ok(Attestation::Electra(AttestationElectra {
+        aggregation_bits,
+        committee_bits,
+        data: single_attestation.data.clone(),
+        signature: single_attestation.signature.clone(),
+    }))
+}