input: Write a python function to generate a random string of a given length containing lowercase letters and digits.
  assert len(generate_random_string(10)) == 10
  assert len(generate_random_string(5)) == 5
  assert len(generate_random_string(15)) == 15
output: import random
  import string
  
  def generate_random_string(length):
      characters = string.ascii_lowercase + string.digits
      return ''.join(random.choice(characters) for _ in range(length))