Add feature: spawn assembly of shapes

[ErcBunny]

Description

Sometimes we want to abstract a robot as a rigid body, manually set its mass, inertia, collision properties, and generate its appearance according to parameters instead of using fixed appearance loaded from files. This PR adds a submodule in side sim.spawner to host scripts for spawning such robots as assemblies of basic shape prims.

Fixes #329 .

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update (spawn_prims.rst is changed)

Screenshots

A quadcopter is spawned additionally when running source/standalone/tutorials/00_sim/spawn_prims.py after the change.

Checklist

• I have run the pre-commit checks with ./orbit.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have run all the tests with ./orbit.sh --test and they pass
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there