input: Write a python function to generate a random string of given length using ascii letters and digits.
  assert generate_random_string(20) != generate_random_string(20)
  assert len(generate_random_string(15)) == 15
  assert all(c in string.ascii_letters + string.digits for c in generate_random_string(10))
output: import random
  import string
  
  def generate_random_string(length):
      characters = string.ascii_letters + string.digits
      return ''.join(random.choice(characters) for _ in range(length))