feat: Test Kit for Servers

[yhl25]

• Test kits for servers to simplify unit testing and component testing
• Capability to run the server locally

Unit Testings

Just test functions without starting neither server or the client

Component Testing - Client code

In this mode, server will be already running.

1. start client
2. using client call the server
3. close the client

      // Create a client that can send requests to the server
       MapperTestKit.Client client = new MapperTestKit.Client();
       MapperTestKit.TestDatum datum = MapperTestKit.TestDatum
               .builder()
               .value("2".getBytes())
               .build();
       MessageList result = client.sendRequest(new String[]{}, datum);
       client.close();

Server code for just testing the UDFs locally

1. start server
2. open client
3. using client invoke the RPCs functions on server
4. close client
5. showdown server

       // Start the server locally. This will initiate the server on localhost and port 50051
       Server server = new Server(new EvenOddFunction());
       // Start the server
       server.start();
       // Wait for the server to shutdown
       server.awaitTermination();

fixes https://github.com/numaproj/numaflow/issues/1625