Add missing license header (opensearch-project#3467)

### Description
Added missing license header for some files.

### Check List
- [ ] 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 <[email protected]>

src/integrationTest/java/org/opensearch/test/framework/testplugins/AbstractRestHandler.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.test.framework.testplugins;
 
 import org.opensearch.ExceptionsHelper;

src/main/java/org/opensearch/security/dlic/rest/validation/EndpointValidator.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.dlic.rest.validation;
 
 import org.opensearch.core.rest.RestStatus;

src/test/java/org/opensearch/security/dlic/rest/api/AccountApiActionConfigValidationsTest.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.dlic.rest.api;
 
 import com.fasterxml.jackson.databind.node.ObjectNode;

src/test/java/org/opensearch/security/dlic/rest/api/ActionGroupsApiActionValidationTest.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.dlic.rest.api;
 
 import org.junit.Before;

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;