[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>

consensus/state_processing/src/per_epoch_processing/altair/sync_committee_updates.rs

@@ -1,5 +1,6 @@
 use crate::EpochProcessingError;
 use safe_arith::SafeArith;
+use std::sync::Arc;
 use types::beacon_state::BeaconState;
 use types::chain_spec::ChainSpec;
 use types::eth_spec::EthSpec;
@@ -12,7 +13,7 @@ pub fn process_sync_committee_updates<T: EthSpec>(
     if next_epoch.safe_rem(spec.epochs_per_sync_committee_period)? == 0 {
         *state.current_sync_committee_mut()? = state.next_sync_committee()?.clone();
 
-        *state.next_sync_committee_mut()? = state.get_next_sync_committee(spec)?;
+        *state.next_sync_committee_mut()? = Arc::new(state.get_next_sync_committee(spec)?);
     }
     Ok(())
 }