input: Write a function to concatenate all elements of a list of strings with a specified separator. The function should return the concatenated string.
  assert concatenate_strings(["Sample", "Text", "Hello"], "-") == "Sample-Text-Hello"
  assert concatenate_strings(["This", "is", "a", "test"], " ") == "This is a test"
  assert concatenate_strings([], "*") == ""
  assert concatenate_strings(["Single"], "/") == "Single"
  assert concatenate_strings(["One", "Two"], "") == "OneTwo"
output: def concatenate_strings(lst, sep):
      return sep.join(lst)