[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/tests/tests.rs

@@ -436,7 +436,8 @@ fn roundtrip_operation_pool() {
         .get_item::<PersistedOperationPool<MinimalEthSpec>>(&OP_POOL_DB_KEY)
         .expect("should read db")
         .expect("should find op pool")
-        .into_operation_pool();
+        .into_operation_pool()
+        .unwrap();
 
     assert_eq!(harness.chain.op_pool, restored_op_pool);
 }