Implement secret key protection using AEAD

[vanitasvitae]

This PR is based on - and blocked by - #1699

Here, I added two new classes, BcAEADSecretKeyEncryptorBuilder and JcaAEADSecretKeyEncryptorBuilder, which can both be used to AEAD-encrypt PGP secret keys.

First I tried to implement AEAD protection into the existing PBESecretKeyEncryptorBuilder classes, but that turned out to result in ugly code. Let me know what you think of my approach.