Better support fixed timesteps in newest Avian

[janhohenheim]

This primarily does the following things.

• Update to breaking Avian changes
  • Notably, physics layers now require an explicit default instead of implicitly belonging to everything when not specified
• Support Avian no longer running in PostUpdate, but in FixedUpdate.
  • The Tnua plugins for Avian no longer have a Default, as the TnuaControllerPlugin needs to be manually set in PhysicsSchedule as well
  • Change some scheduling stuff in examples in order to show idiomatic usage
• Work with Position and Rotation instead of GlobalTransform to better support interpolation between fixed timesteps
  • GlobalTransform is in general just the place where the object in question will be rendered, not where it physically is.
• Fix just_pressed in demos not working reliably with a fixed timestep
  • just_pressed in general does not work out of the box for fixed timesteps. LWIM would handle that, but I didn't want to add a new dependency. So, instead the demo now does its own just_pressed caching.
• Make demos use PhysicsSchedule by default for avian.

I tested and verified that all examples and demos still work.
Some of these changes like the GlobalTransform stuff would be nice for rapier as well, but I'll leave that for a future follow-up PR.

[idanarye]

Wrote some comments. You don't have to fix them - I can fix them myself once I merge after Avian 0.2 is released.

[idanarye]

I think this transform is only used for computing the cast_origin. Maybe we can just compute it directly with the rotation and position instead of creating a transform matrix?

[janhohenheim]

I don't know how we would do that. Creating the transform matrix is super cheap, so I also don't think this is pressing.

[janhohenheim]

Good suggestions! Fixed the things that I knew how to fix :)

[janhohenheim]

Looks like the CI ran into an infinite build. Could you rerun it if you happen to have time? :) Also, I'll try to keep this PR up to date so it's ready for Avian 0.2 when it releases.

[idanarye]

@janhohenheim I've updated the Avian integration crates to Avian 0.2, and since they've officially changed the default schedule I had do these changes on main - otherwise it'd mess up the demos.

I did not implement any of the other things you did in this PR - just the scheduling.