Pass/GPG Backend

[MagicRB]

Clarification and motivation

Implement a new backend, like the already existing one for Vault.Kv that uses either pass or gpg directly. The question is whether to use pass or gpg, they're essentially equivalent, but if we went for gpg we'd avoid a needless dep on pass, I'm not saying that we shouldn't maintain pass compatibilty, but we can go around it if we want.

Acceptance criteria

New functional backend on top of pass/gpp, associated tests, and compatibility with pass.

UPDATE: now having looked at the source code of pass, I refuse to touch GPG with a 20 foot pole, I personally don't mind the extra dependency so I'll just start wrapping pass.