Rework to support usage of encryption subkeys

[Liam-Rougoor]

#238 (partly)

So, I've changed how public keys are managed in EncryptionKeys and how they are used in Encryption and some Verification in PGP.

Summary of the main changes in this PR:

• Every public key ring is now stored as a collection of a new wrapper class - PgpPublicKeyRingWithPreferredKey (I'm still open for better name suggestions). This class stores the public key ring, the default key (using the already existing algorithm to find the 'best' key) and a preferred key, which can be set by the user using EncryptionKeys.UseEncryptionKey(_some_keyId_)
• In the encryption methods, for every known public key ring (a.k.a. recipient), PGP uses the preferred key if it's set, otherwise the default key.
• When verifying encrypted data, it will look for a matching key in any of the known keys in the key rings.

I think more improvements can be made regarding the handling of public keys, but I think this PR could give an indication of a possible direction.

[mattosaurus]

This is looking good. Do we also want to extend it to setting the preferred signing and verification keys?

[Liam-Rougoor]

This is looking good. Do we also want to extend it to setting the preferred signing and verification keys?

That sounds like a good idea. Perhaps something we could implement in a separate PR?

[mattosaurus]

Sure, happy for this to be done in a separate PR.