Add SPDX and OpenSearch Modification license header

Signed-off-by: Shephali Mittal <[email protected]>

test_elasticsearch/test_server/test_rest_api_spec.py

@@ -1,3 +1,12 @@
+# SPDX-License-Identifier: Apache-2.0
+#
+# The OpenSearch Contributors require contributions made to
+# this file be licensed under the Apache-2.0 license or a
+# compatible open source license.
+#
+# Modifications Copyright OpenSearch Contributors. See
+# GitHub history for details.
+#
 #  Licensed to Elasticsearch B.V. under one or more contributor
 #  license agreements. See the NOTICE file distributed with
 #  this work for additional information regarding copyright