input: Write a python function to generate a random string of given length.
  assert len(generate_random_string(10)) == 10
  assert len(generate_random_string(5)) == 5
  assert generate_random_string(0) == ''
output: import random
  import string
  
  def generate_random_string(length):
      return ''.join(random.choices(string.ascii_letters + string.digits, k=length))