Add payload attestation pool and block packing

Store gossip-verified PayloadAttestationMessages in the operation pool
keyed by PayloadAttestationData. At block production time, aggregate
them into PayloadAttestations using PTC position mapping and include
in the block body. Wire pool insertion into the gossip handler after
fork choice.

beacon_node/network/src/network_beacon_processor/gossip_methods.rs

@@ -4156,6 +4156,14 @@ impl<T: BeaconChainTypes> NetworkBeaconProcessor<T> {
                         ),
                     }
                 }
+
+                if let Err(e) = self.chain.add_payload_attestation_to_pool(&verified) {
+                    warn!(
+                        reason = ?e,
+                        %peer_id,
+                        "Failed to add payload attestation to pool"
+                    );
+                }
             }
             Err(RejectedPayloadAttestation {
                 payload_attestation_message,