Add API to list all assets

[rajat2004]

Fixes: #3930

About

• Adds a new simListAssets API, useful with the simSpawnObject API.
• An example script for testing
• C++ wrappers for spawn,destroy object APIs, added since Python APIs were already added in Add Python API for new object functions #2897, and C++ ones were left out in add simLoadLevel, simGet/SetObjectScale, simSpawn|DestroyObject #2651

How Has This Been Tested?

With the example script

Screenshots (if appropriate):

airsim_list_assets.mp4

[zimmy87]

Tested the create_objects.py script locally, it works for me, so I am moving ahead with merging. Thanks for the contribution @rajat2004!