feat(GODT-2576): Forward and $Forward Flag Support

When an IMAP client stores the `Forward` or `$Forward` flags on a
message, the forwarded state is now correctly represented on the Proton
servers.

https://github.com/ProtonMail/go-proton-api/pull/125
https://github.com/ProtonMail/gluon/pull/400

internal/services/imapservice/helpers.go

@@ -68,6 +68,10 @@ func BuildFlagSetFromMessageMetadata(message proton.MessageMetadata) imap.FlagSe
 		flags.AddToSelf(imap.FlagAnswered)
 	}
 
+	if message.IsForwarded {
+		flags.AddToSelf(imap.ForwardFlagList...)
+	}
+
 	return flags
 }