input: Write a python function to find pairs in a list with a specific difference.
  assert find_pair_with_difference([9, 4, 7, 5], 4)==[(5, 9)]
  assert find_pair_with_difference([1, 5, 3, 8, 12], 4)==[(1, 5), (8, 12)]
  assert find_pair_with_difference([10, 15, 20, 25, 30], 5)==[(10, 15), (15, 20), (20, 25), (25, 30)]
output: def find_pair_with_difference(lst, diff):
      pairs = []
      lst_set = set(lst)
      for num in lst:
          if num + diff in lst_set:
              pairs.append((num, num + diff))
      return pairs