build: add release-plz for automating releases

[esteve]

This adds release-plz for automating releases. For a similar PR for rosidl_runtime_rs, check ros2-rust/rosidl_runtime_rs#14

[esteve]

It looks like CI is failing because of colcon/colcon-cargo#63, after adding a toplevel Cargo.toml file for the workspace, rclrs_example_msgs becomes unavailable:

$ colcon info --packages-select rclrs_example_msgs
[0.168s] WARNING:colcon.colcon_core.package_selection:ignoring unknown package 'rclrs_example_msgs' in --packages-select

and this is with the toplevel Cargo.toml file removed:

$ colcon info --packages-select rclrs_example_msgs
path: src/ros2_rust/examples/rclrs_example_msgs
  type: ros.ament_cmake
  name: rclrs_example_msgs
  dependencies:
    build: ament_cmake rosidl_default_generators
    run: rosidl_default_runtime
    test: ament_lint_common
  metadata:
    maintainers: ['Nikolai Morin <nnmmgit@gmail.com>']
    version: 0.4.1

[esteve]

Preliminary fix for the colcon-cargo issue in colcon/colcon-cargo#64

[maspe36]

LGTM, will merge once CI passes

[esteve]

@maspe36 thanks. The ROS APT repo seems to not be fully synced yet, CI is failing to fetch a package that's listed in the repo's Packages file, I've triggered the CI again.