add obstacles to empty world

[mrinalTheCoder]

Description

This PR does the following:

• Adds obstacles like cube and ramp to empty world using launch arguments

Testing Steps (if relevant)

Test Case 1

1. ros2 launch urc_bringup sim.launch.py world:=empty_world.sdf autonomy:=true add_incline:=true incline_slope:=30.0 incline_x:=2.0 incline_y:=3.0
2. ros2 launch urc_bringup sim.launch.py world:=empty_world.sdf autonomy:=true add_cube:=true cube_x:=2.0 cube_y:=3.0
3. You can also add both an incline and a cube, and play around with the parameters to alter positions.

Expectation: Empty world with specified obstacles. Theoretically it should work with the terrain world also, but it might be unstable due to uneven terrain.

[bkuangs]

looks g