Geometry Nodes: Experimental rigid body integration.

This is an exploration of how geometry nodes might be coupled with
rigid bodies and iterative simulations in general. It's a very
rough-and-ready implementation, not meant as a final version, but rather
to prove the possiblity and to find challenging areas where redesign is
needed.

The core additions are:
- Geometry nodes to flag points and/or instances as rigid bodies.
- Depsgraph integration to ensure the necessary order of operations
    between modifiers and rigid body pre/post simulation updates.
- Simple cache feature to store arbitrary geometry and loop back into
    the next iteration.

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