sdk: Make transaction_id truly optional for send and send_raw

… by removing the parameter and returning a named future with a
builder-style `with_transaction_id` method.

examples/command_bot/src/main.rs

@@ -22,9 +22,7 @@ async fn on_room_message(event: OriginalSyncRoomMessageEvent, room: Room) {
         println!("sending");
 
         // send our message to the room we found the "!party" command in
-        // the last parameter is an optional transaction id which we don't
-        // care about.
-        room.send(content, None).await.unwrap();
+        room.send(content).await.unwrap();
 
         println!("message sent");
     }