Graph introspection incomplete when nodes with duplicate names present

[Yadunund]

The current GraphCache implementation assumes nodes within a namespace will have unique names. But this is not necessarily true for ROS 2 nodes. Hence, with two talker nodes running, commands like ros2 node list will return only reveal a single /talker node.

Note: With DDS, GIDs are available for entities which are globally unique. With zenoh, each session will have a unique z_session_id.

Potential solutions:

1. Update liveliness token keyexprs to also include the z_session_id of the entity. Then update NodeMap to be an std::multimap with a custom comparator that sorts based on the z_session_id.

[Yadunund]

Closed by #87