Updates checkstyle to look for incorrect license header

Signed-off-by: Darshit Chanpura <[email protected]>

config/checkstyle/sun_checks.xml

@@ -204,4 +204,12 @@
     <property name="severity" value="warning"/>
   </module>
 
+  <!-- Checks for Headers to not contain old license text -->
+  <module name="RegexpSingleline">
+    <property name="format" value="\*( Portions)? Copyright OpenSearch Contributors"/>
+    <property name="ignoreCase" value="true"/>
+    <property name="message" value="Incorrect license header" />
+    <property name="severity" value="error"/>
+  </module>
+
 </module>

src/main/resources/license_headers/license_header.txt

@@ -0,0 +1,10 @@
+/*
+ * 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.
+ */