Retry custody requests after peer metadata updates (#6975)

Closes https://github.com/sigp/lighthouse/issues/6895

We need sync to retry custody requests when a peer CGC updates. A higher CGC can result in a data column subnet peer count increasing from 0 to 1, allowing requests to happen.


  Add new sync event `SyncMessage::UpdatedPeerCgc`. It's sent by the router when a metadata response updates the known CGC

beacon_node/network/src/service.rs

@@ -485,6 +485,9 @@ impl<T: BeaconChainTypes> NetworkService<T> {
             NetworkEvent::PeerDisconnected(peer_id) => {
                 self.send_to_router(RouterMessage::PeerDisconnected(peer_id));
             }
+            NetworkEvent::PeerUpdatedCustodyGroupCount(peer_id) => {
+                self.send_to_router(RouterMessage::PeerUpdatedCustodyGroupCount(peer_id));
+            }
             NetworkEvent::RequestReceived {
                 peer_id,
                 inbound_request_id,