Add demo for worker feature

Signed-off-by: Michael X. Grey <greyxmike@gmail.com>

examples/worker_demo/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "worker_demo"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+rclrs = "0.4"
+std_msgs = "*"

examples/worker_demo/src/main.rs

@@ -0,0 +1,39 @@
+use rclrs::*;
+use std::time::Duration;
+
+fn main() -> Result<(), RclrsError> {
+    let mut executor = Context::default_from_env()?.create_basic_executor();
+    let node = executor.create_node("worker_demo")?;
+
+    let publisher = node.create_publisher("output_topic")?;
+    let worker = node.create_worker(String::new());
+
+    let _timer = worker.create_timer_repeating(
+        Duration::from_secs(1),
+        move |data: &mut String| {
+            let msg = std_msgs::msg::String {
+                data: data.clone()
+            };
+
+            publisher.publish(msg).ok();
+        }
+    )?;
+
+    let _subscription = worker.create_subscription(
+        "input_topic",
+        move |data: &mut String, msg: std_msgs::msg::String| {
+            *data = msg.data;
+        }
+    )?;
+
+    println!(
+        "Beginning repeater... \n >> \
+        Publish a std_msg::msg::String to \"input_topic\" and we will periodically republish it to \"output_topic\".\n\n\
+        To see this in action run the following commands in two different terminals:\n \
+        $ ros2 topic echo output_topic\n \
+        $ ros2 topic pub input_topic std_msgs/msg/String \"{{data: Hello}}\""
+    );
+    executor.spin(SpinOptions::default());
+
+    Ok(())
+}