input: Write a python function to perform operations 'sum' and 'mean' on a list of numbers.
  assert web_service([1, 2, 3, 4, 5], 'sum') == 15
  assert web_service([1, 2, 3, 4, 5], 'mean') == 3.0
  assert web_service([10, 20, 30, 40], 'sum') == 100
  assert web_service([10, 20, 30, 40], 'mean') == 25.0
output: def web_service(numbers, operation):
      if operation == 'sum':
          return sum(numbers)
      elif operation == 'mean':
          return sum(numbers) / len(numbers)