input: Write a python function to get characters with exact frequency from a given string.
  assert get_chars_with_exact_freq("mississippi", 3) == ['s']
  assert get_chars_with_exact_freq("aabbcc", 2) == ['a', 'b', 'c']
  assert get_chars_with_exact_freq("abcdabcdabcd", 3) == ['a', 'b', 'c', 'd']
output: def get_chars_with_exact_freq(s, freq):
      from collections import Counter
      counter = Counter(s)
      return [char for char, count in counter.items() if count == freq]