add header to the test class

Signed-off-by: Jialiang Liang <jiallian@amazon.com>

Author: RyanL1997

legacy/src/test/java/org/opensearch/sql/legacy/plugin/RestSQLActionTest.java

@@ -1,3 +1,8 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
 package org.opensearch.sql.legacy.plugin;
 
 import static org.mockito.ArgumentMatchers.argThat;