Add support for logging services

[romainreignier]

This is my attempt to fix #642, largely inspired by the parameters services.

A few notes:

• I have gated this feature to avoid humble, the logging services have been added for the Iron release.
• the node option is called enable_logger_service() and not start_logger_services() like for the parameters because I have used the same wording than rclcpp, is that ok or do you prefer to be consistent with the parameter services option?
• I have replaced LogSeverity::from_native() that was used only in the tests to LogSeverity::try_from() that seems more idiomatic to me and not panic on error but returns a custom error.
• I was not sure about how to retrieve the error message from rcutils_logging_set_logger_level() in case of an error to set the reason field of the service response. I must confess that I have followed Claude Code for this:

  let reason = match &err {
      RclrsError::RclError { msg: Some(m), .. }
      | RclrsError::UnknownRclError { msg: Some(m), .. } => m.to_string(),
      other => other.to_string(),
  };

Assisted-by: Claude:claude-4.7-opus