Add backup support for encrypted notes on-chain

[ghost]

No description provided.

[nokitakaze]

I decided not to wait for the public disclosure of the encryption algorithm.
https://github.com/nokitakaze/tornado-cash-encrypted-note.net

[ghost]

@nokitakaze Very impressive, it will be worth to study for newcomers, thank you!

[nokitakaze]

It wasn't too difficult.