input: Write a python function to search for files with a specific extension in a directory.
  assert search_files('./sample_files', '.py') == ['./sample_files/test1.py', './sample_files/test2.py']
  assert search_files('./sample_files', '.txt') == ['./sample_files/readme.txt', './sample_files/data.txt']
  assert search_files('./sample_files', '.md') == ['./sample_files/README.md']
output: def search_files(directory, extension):
      import os
      
      result = []
      for root, dirs, files in os.walk(directory):
          for file in files:
              if file.endswith(extension):
                  result.append(os.path.join(root, file))
      return result