Build time generation of fixed-size strings

[MiguelCompany]

This is part of a PoC towards achieving the goals of ros2/rclcpp#2201

• On rosidl_runtime_cpp, a new class has been added that behaves as a bounded string but can be considered POD by the underlying middleware.
• On rosidl_generator_cpp, processing of a new cdr_plain annotation has been introduced, so string members with that annotation will be generated using the new class.
• On rosidl_adapter, strings can be annotated with this new annotation depending on the contents of a YAML file specified in the environment variable ROS2_TYPE_FIXED_SIZE_FILE.