verify signed messages(detached)

[DenBond7]

it's similar to #1097

[DenBond7]

https://thunderbird.topicbox.com/groups/e2ee/T9f57ded9092e28c8/how-can-i-manually-verify-the-signature-using-gpg-sent-by-thunderbird
https://datatracker.ietf.org/doc/html/rfc3156#section-5
https://www.gnupg.org/gph/en/manual/x135.html

For history: for verification, OpenPGP/MIME signed message is very important to download the MIME message as-is(without any modification). It seems IMAP server returns the MIME message as-is when we fetch a whole message but can modify content(change CR/LF) when we fetch some parts(for example a signed part). After that, we will be not able to verify the signed content. Need to be careful there.

[DenBond7]

ref pgpainless/pgpainless#240 pgpainless/pgpainless#243