Use bulk verification for sync_aggregate signature (#2415)

## Proposed Changes

Add the `sync_aggregate` from `BeaconBlock` to the bulk signature verifier for blocks. This necessitates a new signature set constructor for the sync aggregate, which is different from the others due to the use of [`eth2_fast_aggregate_verify`](https://github.com/ethereum/eth2.0-specs/blob/v1.1.0-alpha.7/specs/altair/bls.md#eth2_fast_aggregate_verify) for sync aggregates, per [`process_sync_aggregate`](https://github.com/ethereum/eth2.0-specs/blob/v1.1.0-alpha.7/specs/altair/beacon-chain.md#sync-aggregate-processing). I made the choice to return an optional signature set, with `None` representing the case where the signature is valid on account of being the point at infinity (requires no further checking).

To "dogfood" the changes and prevent duplication, the consensus logic now uses the signature set approach as well whenever it is required to verify signatures (which should only be in testing AFAIK). The EF tests pass with the code as it exists currently, but failed before I adapted the `eth2_fast_aggregate_verify` changes (which is good).

As a result of this change Altair block processing should be a little faster, and importantly, we will no longer accidentally verify signatures when replaying blocks, e.g. when replaying blocks from the database.

testing/ef_tests/src/cases/operations.rs

@@ -198,7 +198,7 @@ impl<E: EthSpec> Operation<E> for SyncAggregate<E> {
         spec: &ChainSpec,
     ) -> Result<(), BlockProcessingError> {
         let proposer_index = state.get_beacon_proposer_index(state.slot(), spec)? as u64;
-        process_sync_aggregate(state, self, proposer_index, spec)
+        process_sync_aggregate(state, self, proposer_index, VerifySignatures::True, spec)
     }
 }