Cache contacts for sleeping bodies when using Jolt Physics

[mihe]

Reverts #100533.
Fixes #100515.
Fixes #100700.

This pull request does two things:

1. It reverts the changes to jolt_contact_listener_3d.cpp introduced in #100533, due to taking a different approach to solving #100515.
2. It solves #100515 by instead emulating what Godot Physics does, which is to only clear the list of contacts while a body is actually active/awake. This means we're now deliberately keeping (potentially stale) contacts around when a body goes to sleep, in order to better facilitate contact-based logic in scripts, as discussed in #100533.

[mihe]

I've put up another pull request (#100983) to change the way in which sleeping bodies are iterated over when using Jolt, which inadvertently resolves #100700. This means the changes to JoltBody3D in this pull request aren't needed anymore, so I'll put up a new pull request that just reverts #100533 instead.