Add initial implementation of Gopenpgp-backed PGP

[msfjarvis]

📢 Type of change

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

📜 Description

Adds alternate implementations of GPG-handling activities which use Gopenpgp, and a simple feature flag to prevent this code from being exposed end users of snapshots.

💡 Motivation and Context

Work towards #1195

The intent of this PR is to demonstrate viability, and little more. It has no immediate benefits to users, and only serves to create an MVP that must be reworked as and when we start implementing new supporting functionality.

💚 How did you test it?

Verified with hard-coded credentials that encryption and decryption work.

📝 Checklist

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

🔮 Next steps

• Add key management UI
• Decide on whether or not a passphrase cache should be implemented

[msfjarvis]

TODO for myself

• Potentially get rid of the BasePgpActivity superclass. Can happen separately.
• Implement a rudimentary key import mechanism. Being done in Add key management capabilities #1446
• Implement a prompt to ask user for their key passphrase Will be done after Add key management capabilities #1446 is in
• Decide whether we want a passphrase cache and implement it

[msfjarvis]

I'm going to mark this as ready for review since the code here is basically done until #1446 is complete, which'll take some time.

To test this, you'll have to hard-code your PGP private key and passphrase at the bottom of GopenpgpDecryptActivity, and your PGP public key at the bottom of GopenpgpPasswordCreationActivity. The keys must be exported with --armor for now, that may or may not change by the time #1446 is done.