Support for building Debian packages

[ashquarky]

Support for building Debian packages

[esteve]

@ashquarky thanks for your interest in ros2-rust. Adding support for building Debian packages out of ros2-rust is not currently possible, we'd need at least these two before we can do that:

• Add support for Cargo to rosdep. No easy feat, we talked with the maintainers of the ROS buildfarm and they raised several concerns about it,
• Include the Rust toolchain in the ROS Buildfarm. The ROS buildfarm builds packages without access to the internet, as a security measure, we'd need to find a way to "freeze" dependencies so that Cargo does not download them

As a first step, we want to submit rosidl_runtime_rs to the ROS buildfarm, as it's just a regular ROS package written in Python, and eventually enable it as one of the default message generators. Before that we want to add support for actions (see #295), which would bring it to pretty much feature parity with other message generators. Once we have that included in the standard ROS distribution, all messages will be generated as Rust structs and although rclrs will still be distributed as a crate, the barrier to entry for having ROS 2 applications written in Rust will be much lower.