Node traits

Description

I would find handy to have in rclcpp C++ traits like (draft):

rclcpp::is_node<typename T>
rclcpp::is_node_v<typename T>
rclcpp::is_lifecycle_node<typename T>
rclcpp::is_lifecycle_node_v<typename T>

they can be easily implemented via std::is_base_of.

the is_node trait may be ambiguous. We can:

• return true for both rclcpp::Node and rclcpp::LifecycleNode (and use the is_lifecycle_node to further discriminate among the two)
• return true ONLY for rclcpp::Node

I see there are traits for other things like subscriptions and so on in the library, but before drafting a PR I'd like some feedback (or tell me if it is something it's better not to put in rclcpp).

Motivation

Mainly, this would help distinguish between normal nodes and lifecycle nodes, write better templates and so on.

Design / Implementation Considerations

No response

Additional Information

No response

Review the existing rclcpp traits for subscriptions and the definitions of Node and LifecycleNode. Resolve whether is_node should include lifecycle nodes, then determine the appropriate public trait location and coverage. Done means the trait behavior is unambiguous and the requested normal-node and lifecycle-node distinctions are supported.