input: Write a python function to return a list of Armstrong numbers within a given range.
  assert armstrong_numbers(1, 100) == [1, 2, 3, 4, 5, 6, 7, 8, 9]
  assert armstrong_numbers(100, 500) == [153, 370, 371, 407]
  assert armstrong_numbers(1, 1000) == [1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407]
output: def armstrong_numbers(start, end):
      def is_armstrong(num):
          digits = list(map(int, str(num)))
          power = len(digits)
          return sum(digit ** power for digit in digits) == num
  
      return [num for num in range(start, end) if is_armstrong(num)]