feat: Add a standalone CLI program for decrypting 2fas files

[elliotwutingfeng]

Feature description

• An official standalone CLI program to decrypt encrypted *.2fas files, written in a common programming language like Python, Ruby, Go, Java, Kotlin, Swift etc. Dynamic scripting languages preferred. The program need not pretty-print the plaintext output; just the raw JSON will do. Any further formatting should be the end-user's responsibility.

Motivation

In a recent Discord conversation, it was mentioned that users are not "locked in" to 2FAS as the *.2fas backup file generation methods are not secret.

While this is technically true because 2FAS is open source, a user who wants to decrypt the backup file will need to spend time and effort reading source code and implementing decryption methods, or rely on third-party developers to do it for them, as evident in #117.

Other FOSS authenticator apps solve this problem by providing official scripts for decrypting backups. In Aegis, they have this Python script, and in Ente, their official CLI tool can decrypt their own backup files directly.

Acknowledgements

• This issue is not a duplicate of an existing feature request.
• I have chosen an appropriate title.
• All requested information has been provided properly.