Add tags() method that returns Option<Arc<RwLock<BTreeMap<u64, String>>>>.
The BTreeMap holds available tags in a mail backend and uses the tag's
hash as key.
The method returns an Option because not all backends may support
tagging.
fetch BODYSTRUCTURE along with ENVELOPE from server and set
has_attachments based on the MIME structure of the envelope.
Notes: BODYSTRUCTURE returns the MIME structure of the envelope without
the data, so if it includes a multipart/mixed it *should* have
attachments.
ENVELOPE returns basic headers of the message like Sender, Subject, Date
etc.
Attachment needs to know the range of bytes where the body part of the
attachment is located. The Attachment.raw field contains the entire
attachment, headers and body. The new Attachment.body fields contains a
`StrBuilder` which contains the offset and length of the body part inside
`raw`.
Check subattachments in has_attachments check.
Instead of getting a flattened attachment view of multipart/mixed (eg
[multipart/mixed, text/plain, text/plain]) get only the subattachments
(eg [text/plain, text/plain]). Don't count text-only multipart/mixed as attachments
Manos Pitsidianakis