Extend SubscriptionCallback trait to ReadOnlyLoanedMessage (#266)

This was not a straightforward extension because
1. ReadOnlyLoanedMessage has a lifetime
2. The take_loaned_message() method was restricted to Subscription<T: RmwMessage>, i.e. not available
   for subscriptions on idiomatic messages.
   This has been fixed and now the method is implemented for Subscription<T: Message> like the other
   `take*` methods – though you still will get an RMW-native loaned message even if T is the idiomatic
   message type, because idiomatic message types can never be loaned.

Author: nnmm

rclrs/minimal_pub_sub/src/zero_copy_subscriber.rs

@@ -12,7 +12,7 @@ fn main() -> Result<(), Error> {
     let _subscription = node.create_subscription::<std_msgs::msg::UInt32, _>(
         "topic",
         rclrs::QOS_PROFILE_DEFAULT,
-        move |msg: std_msgs::msg::UInt32| {
+        move |msg: rclrs::ReadOnlyLoanedMessage<'_, std_msgs::msg::UInt32>| {
             num_messages += 1;
             println!("I heard: '{}'", msg.data);
             println!("(Got {} messages so far)", num_messages);