perf(v5): replace copy_from_slice in Publish::new and LastWill::new

Both used `Bytes::copy_from_slice`, which allocates a String and copies
its bytes into a second Bytes allocation before dropping the String.
Replace with `Bytes::from` to take ownership of the String's buffer
directly — same approach as the v4 Publish constructor — cutting the
topic allocation count from 2, down to 1 per publish.

Author: de-sh

rumqttc/src/v5/mqttbytes/v5/connect.rs

@@ -371,7 +371,7 @@ impl LastWill {
         retain: bool,
         properties: Option<LastWillProperties>,
     ) -> LastWill {
-        let topic = Bytes::copy_from_slice(topic.into().as_bytes());
+        let topic = Bytes::from(topic.into().into_bytes());
         LastWill {
             topic,
             message: Bytes::from(payload.into()),

rumqttc/src/v5/mqttbytes/v5/publish.rs

@@ -20,7 +20,7 @@ impl Publish {
         payload: P,
         properties: Option<PublishProperties>,
     ) -> Self {
-        let topic = Bytes::copy_from_slice(topic.into().as_bytes());
+        let topic = Bytes::from(topic.into().into_bytes());
         Self {
             qos,
             topic,