support keys with custom/user-specified passphrases

[azul]

https://autocrypt.org/level1.html#secret-key-generation-and-storage has some specification about how to handle password protection of keymaterial.

@hpk42 said in #27 :

My general take on if people want to use e.g. keys with passphrases is that they do the key generation with gpg themselves and then do muacrypt add-account --use-key KEYHANDLE --gpgmode=system to select that key. this way they could aslo use keys from smart cards etc. however, i think that the current decrypt/encrypt API hardcodes passphrases and thus will fail for that case. I am not sure if we need to support passphrases, it's also a feature that can come later as the autocrypt spec does not require support for keys with passphrases.