input: Write a python function to concatenate a list of strings using a given separator.
  assert concatenate_strings(["hello", "world"], ", ") == "hello, world"
  assert concatenate_strings(["a", "b", "c"], "-") == "a-b-c"
  assert concatenate_strings(["apple", "banana", "cherry"], " ") == "apple banana cherry"
output: def concatenate_strings(strings, separator):
      return separator.join(strings)