Add --account-keys option

[xecrets]

Take a file serialized as a UserAccounts instance, containing a list of UserAccount instances, which in turn contains a keypair, where the private key is AxCrypt-encrypted with a password and base64-encoded.

Add the public keys to the list of loaded public keys, but only the most recent for a given email, and only if that is not already in the list of public keys.

Use the passwords to try to decrypt the private keys, and add the ones that can be decrypted to the LogOnIdentity that could decrypt it similar to how UseKeyPairOperation does it.

Optionally, if a second file is given, also write out an updated version of the account keys, with all decryptable private keys re-encrypted with the primary (first) password provided.