input: Write a function that takes a list of strings and returns the concatenation of all the elements with a specified separator.
  
  Input:
  strings = ["Sample","Text","Hello"]
  separator = "-"
  
  Output:
  def concatenate_strings(strings, separator):
   return separator.join(strings)
  
  assert concatenate_strings(["Sample","Text","Hello"], "-") == "Sample-Text-Hello"
  assert concatenate_strings(["This", "is", "a", "test"], " ") == "This is a test"
output: def concatenate_strings(strings, separator):
      return separator.join(strings)