This PR returns an error if cargo-ament-build was not found, including a message on how to install it, it's pretty straightforward but a few notes:

• I opted to recommend for using cargo install cargo-ament-build as noted in the ros2-rust README, I thought about recommending pip but then it would cause a lot of trouble on 24.04 because virtual envs etc.
• I thought of falling back to a colcon-cargo package but I'm not 100% convinced it is a good idea to silently continue building with a build type different from what the users might expect, instead of loudly printing an error and failing.
• The failure method of this package identification is a bit odd and I'm not 100% sure it is the right thing to do, specifics below in the Test it! section.

Test it!

• Copy the sample package into a colcon workspace
• Try a colcon build with cargo ament-build installed
• Remove cargo-ament-build
• Try again

You should see the (as I mentioned above slightly strange) failure:

lucadv@ros:~/colcon_ws$ colcon build
[0.241s] ERROR:colcon.colcon_ros_cargo.package_identification.ament_cargo:ament_cargo package found but cargo ament-build was not detected. Please install it by running: `cargo install cargo-ament-build`
[0.332s] WARNING:colcon.colcon_core.verb:No task extension to 'build' a 'ros.ament_cargo' package

Specifically the second warning is a bit odd but it will happen in all our error paths, so we should probably do something more than early returning on failure, but that is something out of scope for this specific PR