feat(rapier_testbed): autosave testbed configuration + add support for per-example settings

[sebcrozet]

Improve the ergonomics of the rapier testbed by:

• Automatically saving the example selection, camera position, example settings, etc. and restore them when starting the testbed app again.
• Add support for per-example settings. For example in the video below, the number of cubes in the pyramid can be controlled (this the only example modified to use that new feature for now):

pyramid_settings.mov