Add missing license header (#3467)

Added missing license header for some files.

- [ ] New functionality includes testing
- [ ] New functionality has been documented
- [ ] Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and
signing off your commits, please check
[here](https://github.com/opensearch-project/OpenSearch/blob/main/CONTRIBUTING.md#developer-certificate-of-origin).

Signed-off-by: Andrey Pleskach <ples@aiven.io>
(cherry picked from commit fb82bbb)

Author: willyborankin

src/test/java/org/opensearch/security/transport/SecuritySSLRequestHandlerTests.java

@@ -1,3 +1,12 @@
+/*
+ * Copyright OpenSearch Contributors
+ * 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.
+ *
+ */
 package org.opensearch.security.transport;
 
 import org.junit.Assert;