Add initial support for calling ROS2 services from Rust nodes

[phil-opp]

• Implement message generation for ROS2 services in Rust API
• Update rust-ros2-dataflow example with ROS2 service call

Notes

• I also tried to use the synchronous service API of the ros2-client crate, but I ran into issues (see Issues with ros2_service_client example Atostek/ros2-client#24)
• The service detection requires a background event loop provided by the Spinner type. This spinner needs to be run in parallel to the main thread. The easiest way to do that is to create a multi-threaded executor and spawn it there. For example, using the ThreadPool executor provided by the futures crate:

  let pool = futures::executor::ThreadPool::new()?;
  
  let spinner = ros_node.spinner();
  pool.spawn(async {
      if let Err(err) = spinner.spin().await {
          eprintln!("ros2 spinner failed: {err:?}");
      }
  })
  .context("failed to spawn ros2 spinner")?;

  If you're already using an async runtime such as tokio or smol, you can spawn the spinner there as well (provided that you're using a multi-threaded executor).
• The generated type format might still change because we want to support service calls from C++ nodes as well. The cxx crate, which we use for bridging Rust to C++ code, doesn't work with generics, so we have to find another way.
• We might want to provide direct service support in our node API. For example, we might want to spawn the spinner thread already in the library or provide a way to merge service responses into the event stream.

[phil-opp]

(There was an issue that the latest commit didn't show up in the PR. I fixed that issue by temporarily switching the base branch.)

[haixuanTao]

Thank you!