Add example for dynamic messages

rclrs/dynamic_pub_sub/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "dynamic_pub_sub"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+# TODO(luca) change this to the correct version once dynamic message support is released on crates.io
+rclrs = { version = "0.4", features = ["dyn_msg"] }
+anyhow = {version = "1", features = ["backtrace"]}

rclrs/dynamic_pub_sub/src/main.rs

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+use anyhow::{Error, Result};
+use rclrs::*;
+
+fn main() -> Result<(), Error> {
+    let context = Context::default_from_env()?;
+    let mut executor = context.create_basic_executor();
+
+    let node = executor.create_node("dynamic_subscriber")?;
+
+    let worker = node.create_worker::<usize>(0);
+    let _subscription = worker.create_dynamic_subscription(
+        "rclrs_example_msgs/msg/VariousTypes".try_into()?,
+        "topic",
+        move |num_messages: &mut usize, msg, _msg_info| {
+            *num_messages += 1;
+            println!("#{} | I heard: '{:#?}'", *num_messages, msg.structure());
+        },
+    )?;
+
+    println!("Waiting for messages...");
+    executor.spin(SpinOptions::default()).first_error()?;
+    Ok(())
+}