Add button to enable/disable interpolation for physics_in_fixed_timestep example

## What problem does this solve or what need does it fill?

The [physics_in_fixed_timestep](https://github.com/bevyengine/bevy/blob/main/examples/movement/physics_in_fixed_timestep.rs) example is really good at showing *how* to run physics in FixedUpdate and interpolate in Update, but it doesn't show *why* you should do this.

## What solution would you like?

We should add a button to toggle interpolation on/off so it's easy to see why we want to use interpolation.