chore: add SPDX-License-Identifier

Author: lidel

client/LICENSE

@@ -2,3 +2,5 @@ The client module is dual licensed under MIT and Apache-2.0.
 
 MIT: https://www.opensource.org/licenses/mit
 Apache-2.0: https://www.apache.org/licenses/LICENSE-2.0
+
+SPDX-License-Identifier: Apache-2.0 OR MIT