input: Write a function that takes a list of strings and returns the concatenation of all the elements with a specified separator/delimiter. The function should be named `concatenate_strings` and it should take two parameters: `strings` (a list of strings) and `separator` (a string that will be used as the delimiter). The function should use the `join` method to concatenate the strings with the given separator. If the input list is empty, the function should return an empty string.
  
  assert concatenate_strings(["Sample","Text","Hello"], " ") == "Sample Text Hello"
  assert concatenate_strings(["Sample","Text","Hello"], "-") == "Sample-Text-Hello"
  assert concatenate_strings(["a","b","c"], "") == "abc"
  assert concatenate_strings([], ",") == ""
output: def concatenate_strings(strings, separator):
      return separator.join(strings)