Add support for PGP/Inline

Continuing https://github.com/deltachat/deltachat-core-rust/issues/2029 discussion here.

Currently Delta Chat only supports PGP/MIME messages, but not PGP/Inline. Support for parsing and decryption of inline PGP messages is needed for interoperability with clients like FlowCrypt which can’t send MIME messages.

It is unclear how to display encryption status of such messages, because inline PGP has security problems documented in https://debian-administration.org/users/dkg/weblog/108, https://dkg.fifthhorseman.net/notes/inline-pgp-harmful/, https://josefsson.org/inline-openpgp-considered-harmful.html. Such messages are not considered to be “encrypted” in Autocrypt At least they can be decrypted and presented without a padlock, like plaintext messages.