[Altair] Sync committee pools (#2321)

Add pools supporting sync committees:
- naive sync aggregation pool
- observed sync contributions pool
- observed sync contributors pool
- observed sync aggregators pool

Add SSZ types and tests related to sync committee signatures.

Co-authored-by: Michael Sproul <michael@sigmaprime.io>
Co-authored-by: realbigsean <seananderson33@gmail.com>

beacon_node/beacon_chain/src/errors.rs

@@ -3,7 +3,7 @@ use crate::beacon_fork_choice_store::Error as ForkChoiceStoreError;
 use crate::eth1_chain::Error as Eth1ChainError;
 use crate::migrate::PruningError;
 use crate::naive_aggregation_pool::Error as NaiveAggregationError;
-use crate::observed_attestations::Error as ObservedAttestationsError;
+use crate::observed_aggregates::Error as ObservedAttestationsError;
 use crate::observed_attesters::Error as ObservedAttestersError;
 use crate::observed_block_producers::Error as ObservedBlockProducersError;
 use futures::channel::mpsc::TrySendError;
@@ -14,7 +14,7 @@ use state_processing::{
     block_signature_verifier::Error as BlockSignatureVerifierError,
     per_block_processing::errors::{
         AttestationValidationError, AttesterSlashingValidationError, ExitValidationError,
-        ProposerSlashingValidationError,
+        ProposerSlashingValidationError, SyncCommitteeMessageValidationError,
     },
     signature_sets::Error as SignatureSetError,
     state_advance::Error as StateAdvanceError,
@@ -60,6 +60,7 @@ pub enum BeaconChainError {
     },
     CannotAttestToFutureState,
     AttestationValidationError(AttestationValidationError),
+    SyncCommitteeMessageValidationError(SyncCommitteeMessageValidationError),
     ExitValidationError(ExitValidationError),
     ProposerSlashingValidationError(ProposerSlashingValidationError),
     AttesterSlashingValidationError(AttesterSlashingValidationError),
@@ -121,10 +122,12 @@ pub enum BeaconChainError {
         old_slot: Slot,
         new_slot: Slot,
     },
+    AltairForkDisabled,
 }
 
 easy_from_to!(SlotProcessingError, BeaconChainError);
 easy_from_to!(AttestationValidationError, BeaconChainError);
+easy_from_to!(SyncCommitteeMessageValidationError, BeaconChainError);
 easy_from_to!(ExitValidationError, BeaconChainError);
 easy_from_to!(ProposerSlashingValidationError, BeaconChainError);
 easy_from_to!(AttesterSlashingValidationError, BeaconChainError);