cpp-example: run as a service

cpp-example: run as a service

Extend the C++ example to run as systemd/SysV services

This change adds service capability to the existing C++ example without
modifying its original behavior. The example can now run either as:
- One-shot executables (existing behavior)
- Long-running services via systemd or SysV init

The service runs as an unprivileged user/group, demonstrating security
best practices for service development. This introduces additional
complexity to the build process, particularly around proper pseudo usage
in development builds. The implementation includes:
- Service configuration files (systemd .service and SysV init script)
- Dedicated user/group creation with appropriate permissions
- JSON configuration file for runtime customization, owned by the
  service user
- Command-line --endless flag to enable service mode
- Full support for both CMake and Meson build systems

This enhancement enables testing debugger configurations that attach to
running processes, expanding the examples' utility for development tools.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>