chore: add missing license headers

Author: Jannis

common/src/subgraph_client/mod.rs

@@ -1,3 +1,6 @@
+// Copyright 2023-, GraphOps and Semiotic Labs.
+// SPDX-License-Identifier: Apache-2.0
+
 mod client;
 mod monitor;
 

common/src/subgraph_client/monitor.rs

@@ -1,3 +1,6 @@
+// Copyright 2023-, GraphOps and Semiotic Labs.
+// SPDX-License-Identifier: Apache-2.0
+
 use std::time::Duration;
 
 use eventuals::{timer, Eventual, EventualExt};