expand_topic_or_service_name should use get_effective_namespace()

Feature request

Feature description

sub_node concept is not being correctly used in some packages that use the function expand_topic_or_service_name(). They use it with get_namespace() to create publishers, instead of get_effective_namespace(). Probably many people do not know the existence of this method or the concept of subnode. For example, Here. If this node is a sub_node (created with create_sub_node(), then the effect of the subnode does not take effect. For:

    color_subnode_ = create_sub_node("rgb");
    image_transport::ImageTransport color_it(color_subnode_);
    pub_color_ = color_it.advertiseCamera("image", 1);

The effect is:

$ ros2 topic list
/camera_info
/image
/image/compressed
/image/compressedDepth
/image/theora

Instead of adding "/rgb" in front of each topic name.

Maybe expand_topic_or_service_name() should receive the rclcpp::Node, and internally, it could call get_effective_namespace().

I can work on it, but I would like to know if you think it's the right solution.

Implementation considerations

Start by reading expand_topic_or_service_name() and get_effective_namespace(), then compare their use in image_transport/src/camera_publisher.cpp. Determine the appropriate API change so a publisher created from a create_sub_node("rgb") context prefixes topic names with /rgb, and verify the affected callers preserve that namespace behavior.