Refactor Jolt-related project settings to only be read as-needed

[mihe]

As suggested in #101237, this refactors the JoltProjectSettings class to not just be a class of getters that themselves just call GLOBAL_GET (which in some cases also get cached as a static variable) but instead a class of actual values.

This is then coupled with ProjectSettings::get_singleton()->connect("settings_changed", ...); to read the settings again whenever they're changed, ensuring that we have up-to-date values without needing to call GLOBAL_GET all the time.

This might perhaps provide a tiny performance boost, but mostly just cleans things up a bit and lets us get rid of the caveat in the documentation about certain settings only being read once.

It should probably be noted that bounce_velocity_threshold can now technically be read from the physics thread (when using physics/3d/run_on_separate_thread) while the main thread is writing to it, but given that it's a float I would assume that there are no concern about "torn reads" or anything like that, and having the most up-to-date value when reading it isn't really a concern in this case.