Convert the supportedModes method to default

Author: tharikaGitHub

components/apimgt/org.wso2.carbon.apimgt.api/src/main/java/org/wso2/carbon/apimgt/api/model/GatewayAgentConfiguration.java

@@ -20,6 +20,7 @@
 
 import org.wso2.carbon.apimgt.api.APIManagementException;
 
+import java.util.ArrayList;
 import java.util.List;
 
 /**
@@ -81,5 +82,7 @@ public interface GatewayAgentConfiguration {
      * This method returns the supported gateway modes for a given gateway type
      * @return List of supported gateway modes
      */
-    List<String> getSupportedModes();
+    default List<String> getSupportedModes() {
+        return new ArrayList<>();
+    }
 }