Add pathlib-style handling of entity path

[abey79]

#4595 introduced an API to escape entity path in the Python SDK. We should follow this thread and go for the more modern and familiar pathlib.Path approach:

rr.EntityPath("world") / "robot"
rr.EntityPath.root() / "world" / file_name / "some string"

Log operations could be added as helper method:

entity = rr.EntityPath("world") / "camera"
entity.log(rr.Points3D(...))

Overall, this would give a very Pythonic look:

world = rr.EntityPath("world")
world.log(rr.Transform(....)

for stuff in my_world:
    entity = world / stuff.name()
    entity.log(rr.Points3D(...))