Update TIdSMTP and TIdSMTPRelay to support sending a TStream instead of a TIdMessage

[rlebeau]

The TIdSMTPServer.OnMsgReceive event provides a client's raw email data in a TStream (with Received: header added to it). For relaying purposes, it would be useful to be able to send this TStream as-is to the next SMTP server, instead of having to parse the TStream into a TIdMessage first just to re-generate it. This can potentially alter/damage the original email.