imap-send: add PLAIN authentication method to OpenSSL

imap-send: add PLAIN authentication method to OpenSSL

The current implementation for PLAIN in imap-send works just fine
if using curl, but if attempted to use for OpenSSL, it is treated
as an invalid mechanism. The default implementation for OpenSSL is
IMAP LOGIN command rather than AUTH PLAIN. Since AUTH PLAIN is
still used today by many email providers in form of app passwords,
lets add an implementation that can use AUTH PLAIN if specified.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>