I want to implement a simple pathfinding algorithm (A star) on RVIZ

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RVIZ isn't a simulator; it's a visualizer and, in the case of interactive markers, it can be used as an user interface, but it's primarily designed to display data from other sources.

If you're interested in path planning, you should write a node or a set of nodes which contain a map representation, do some planning on that map, and publish the things that you would like to visualize as ROS topics.

A few of the message types that RVIZ can visualize natively are occupancy grids (nav_msgs/OccupancyGrid), paths (nav_msgs/Path), poses (geometry_msgs/Pose), and interactive markers.