Fix ssz formatting for /light_client/updates beacon API endpoint (#7806)

#7759


  We were incorrectly encoding the full response from `/light_client/updates` instead of only encoding the light client update

common/eth2/src/lib.rs

@@ -981,6 +981,32 @@ impl BeaconNodeHttpClient {
         })
     }
 
+    /// `GET beacon/light_client/updates`
+    ///
+    /// Returns `Ok(None)` on a 404 error.
+    pub async fn get_beacon_light_client_updates_ssz<E: EthSpec>(
+        &self,
+        start_period: u64,
+        count: u64,
+    ) -> Result<Option<Vec<u8>>, Error> {
+        let mut path = self.eth_path(V1)?;
+
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("beacon")
+            .push("light_client")
+            .push("updates");
+
+        path.query_pairs_mut()
+            .append_pair("start_period", &start_period.to_string());
+
+        path.query_pairs_mut()
+            .append_pair("count", &count.to_string());
+
+        self.get_bytes_opt_accept_header(path, Accept::Ssz, self.timeouts.default)
+            .await
+    }
+
     /// `GET beacon/light_client/bootstrap`
     ///
     /// Returns `Ok(None)` on a 404 error.