[feature] Support for child entities enacting force on parent `RigidBody` about bevy_xpbd HOT 2 OPEN

ExternalForceParent feels too niche for me. There are other ways to achieve the desired behavior, and I don't like introducing a ton of new yet similar components for something so specific.

apply_force_at_point is given the local center of mass. So yes, it should take it into account, assuming the given center of mass is correct.

In your case, I would either manually implement the logic that accumulates the child forces to the parent's ExternalForce, taking into account their offsets, or use joints. You could use a DistanceJoint or some other joint type to align the thrusters etc. to specific offsets (with local anchor points) relative to the actual ship. This way, applying forces on the thrusters (and other connected bodies) should automatically apply forces and torques to the ship correctly.

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For my specific use case, I manually implemented a solution here: https://github.com/ActuallyHappening/bevy_xpbd3d_parenting
It does rely on clear_forces_and_impulses being public, which is the reason for #257
It solves this problem by introducing a new component: InternalForce. I believe a better approach would be to add a second flag to ExternalForce (and ExternalTorque if desired, although I only really care about forces for the moment) .internal, which a dedicated system would propagate to parents as done in my repo link above.

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