Add LMSReplyType method to the EML builder that sets the
X-LMS-Reply-Type header. This allows reply, reply-all, and forward
shortcuts to tag the EML with REPLY or FORWARD so that downstream
services can label the original message accordingly.
Call LMSReplyType("REPLY") in mail_reply.go and mail_reply_all.go,
and LMSReplyType("FORWARD") in mail_forward.go, immediately after
the existing LMSReplyToMessageID call.