[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/operation_pool/src/sync_aggregate_id.rs

@@ -0,0 +1,21 @@
+use serde_derive::{Deserialize, Serialize};
+use ssz_derive::{Decode, Encode};
+use types::{Hash256, Slot};
+
+/// Used to key `SyncAggregate`s in the `naive_sync_aggregation_pool`.
+#[derive(
+    PartialEq, Eq, Clone, Hash, Debug, PartialOrd, Ord, Encode, Decode, Serialize, Deserialize,
+)]
+pub struct SyncAggregateId {
+    pub slot: Slot,
+    pub beacon_block_root: Hash256,
+}
+
+impl SyncAggregateId {
+    pub fn new(slot: Slot, beacon_block_root: Hash256) -> Self {
+        Self {
+            slot,
+            beacon_block_root,
+        }
+    }
+}