input: Write a python function to convert a date string in the format "dd/mm/yyyy" to a Unix timestamp.
  assert convert_to_unix_time_stamp("01/01/1970") == 0.0
  assert convert_to_unix_time_stamp("01/01/2020") == 1577836800.0
  assert convert_to_unix_time_stamp("31/12/1999") == 946684800.0
output: def convert_to_unix_time_stamp(date_string):
      from datetime import datetime
      dt = datetime.strptime(date_string, "%d/%m/%Y")
      return dt.timestamp()