Switch new PGP backend to use PGPainless

[msfjarvis]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring
• Dependency updates

📜 Description

Implements a new crypto backend based on PGPainless, a BouncyCastle-backed, pure-Java library for OpenPGP that irons out a lot of the warts with other PGP libraries and is in active maintenance.

💡 Motivation and Context

While Gopenpgp is great in of itself, it has certain characteristics that make it rather undesirable for us.

1. It uses JNI, which incurs a library loading runtime penalty on the app as well as overhead on each call to a Gopenpgp method.
2. It's impossible to test without an emulator, which significantly slows down our CI.
3. It adds maintenance overhead for us, since we need to maintain a fork of the library to add convenience methods for our use case.

PGPainless gets rid of all these problems:

1. It's a pure Java library built on top of BouncyCastle, just like OpenKeychain
2. Since it does not use JNI, we can unit test it.
3. It is actively maintained and the development of it is sponsored by a corporate entity.

Additionally, it leverages strong typing to entirely prevent multiple bug classes such as using a private and public key interchangeably.

Once this PR is merged I will archive our fork of Gopenpgp.

💚 How did you test it?

Inserted hard-coded credentials and verified decryption/encryption works perfectly.

📝 Checklist

• I formatted the code ./gradlew spotlessApply
• I reviewed submitted code
• I added a CHANGELOG entry if applicable

🔮 Next steps

• Improve the logic for finding matching keys
• Add key import flow
• Add UI to request passphrase at runtime

[Skrilltrax]

LGTM