input: Write a python function to implement a gRPC client that communicates with the HelloWorldServicer server.
  assert greet_user('Alice') == 'Hello Alice'
  assert greet_user('Bob') == 'Hello Bob'
  assert greet_user('Charlie') == 'Hello Charlie'
output: def greet_user(name):
      import grpc
      from helloworld_pb2_grpc import HelloWorldStub
      from helloworld_pb2 import HelloRequest
  
      # Establish a connection to the gRPC server
      channel = grpc.insecure_channel('localhost:50051')
      stub = HelloWorldStub(channel)
  
      # Create a request with the user's name
      request = HelloRequest(name=name)
  
      # Make the call to the server and get the response
      response = stub.SayHello(request)
  
      return response.message