chore: add IsReady of MapStream (#147)

Signed-off-by: Yashash H L <[email protected]>

Author: yhl25

src/main/java/io/numaproj/numaflow/mapstreamer/Service.java

@@ -1,5 +1,6 @@
 package io.numaproj.numaflow.mapstreamer;
 
+import com.google.protobuf.Empty;
 import io.grpc.Status;
 import io.grpc.stub.StreamObserver;
 import io.numaproj.numaflow.map.v1.MapGrpc;
@@ -89,6 +90,17 @@ public void onCompleted() {
         };
     }
 
+    /**
+     * IsReady is the heartbeat endpoint for gRPC.
+     */
+    @Override
+    public void isReady(
+            Empty request,
+            StreamObserver<MapOuterClass.ReadyResponse> responseObserver) {
+        responseObserver.onNext(MapOuterClass.ReadyResponse.newBuilder().setReady(true).build());
+        responseObserver.onCompleted();
+    }
+
     // Construct a HandlerDatum from a MapRequest
     private HandlerDatum constructHandlerDatum(MapOuterClass.MapRequest d) {
         return new HandlerDatum(