add validate rest handler

Signed-off-by: Frank Lou <mloufra@amazon.com>

src/main/java/org/opensearch/ad/AnomalyDetectorExtension.java

@@ -10,6 +10,7 @@
 
 import org.opensearch.ad.rest.RestCreateDetectorAction;
 import org.opensearch.ad.rest.RestGetDetectorAction;
+import org.opensearch.ad.rest.RestValidateDetectorAction;
 import org.opensearch.ad.settings.AnomalyDetectorSettings;
 import org.opensearch.ad.settings.EnabledSetting;
 import org.opensearch.client.opensearch.OpenSearchClient;
@@ -45,7 +46,7 @@ public ExtensionSettings getExtensionSettings() {
 
     @Override
     public List<ExtensionRestHandler> getExtensionRestHandlers() {
-        return List.of(new RestCreateDetectorAction(), new RestGetDetectorAction());
+        return List.of(new RestCreateDetectorAction(), new RestGetDetectorAction(), new RestValidateDetectorAction());
     }
 
     @Override

src/main/java/org/opensearch/ad/rest/RestValidateDetectorAction.java

@@ -0,0 +1,44 @@
+package org.opensearch.ad.rest;
+
+import static org.opensearch.rest.RestRequest.Method.POST;
+import static org.opensearch.rest.RestStatus.NOT_FOUND;
+import static org.opensearch.rest.RestStatus.OK;
+
+import java.util.List;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.opensearch.ad.constant.CommonErrorMessages;
+import org.opensearch.ad.settings.EnabledSetting;
+import org.opensearch.extensions.rest.ExtensionRestRequest;
+import org.opensearch.extensions.rest.ExtensionRestResponse;
+import org.opensearch.rest.RestHandler.Route;
+import org.opensearch.rest.RestRequest.Method;
+import org.opensearch.sdk.ExtensionRestHandler;
+
+public class RestValidateDetectorAction implements ExtensionRestHandler {
+    private final Logger logger = LogManager.getLogger(RestValidateDetectorAction.class);
+
+    @Override
+    public List<Route> routes() {
+        return List.of(new Route(POST, "/detectors"));
+    }
+
+    @Override
+    public ExtensionRestResponse handleRequest(ExtensionRestRequest request) {
+        if (!EnabledSetting.isADPluginEnabled()) {
+            throw new IllegalStateException(CommonErrorMessages.DISABLED_ERR_MSG);
+        }
+        Method method = request.method();
+
+        if (!Method.POST.equals(method)) {
+            return new ExtensionRestResponse(
+                request,
+                NOT_FOUND,
+                "Extension REST action improperly configured to handle " + request.toString()
+            );
+        }
+        // do things with request
+        return new ExtensionRestResponse(request, OK, "placeholder");
+    }
+}