feat: at_file_share demo

[murali-shris]

- What I did

• created a demo package to test file send and receive through StorJ - How I did it
• file_sender.dart
  • read a file from local path
  • call at_client encryption service,
  • upload encrypted file to StorJ
  • send notification to receiver
• file_receiver.dart
  • listen to notification
  • get file url and other details from notification
  • down encrypted file from Storj
  • decrypt encrypted file using at_client encryption service
  • store decrypted file to local path - How to verify it
  • Start local virtual environment
• follow instructions in README.md