Allow users to override SkeletonModifier3D._process_modification

[lyuma]

Currently, SkeletonModifer3D is abstract. This changes it so that it and _process_modification can be implemented.

This is needed so users can build their own custom IK, spring bones or body tracking.

[lyuma]

The main point of this PR is allowing custom modifiers with virtual _process_modification (with underscore) and I am happy with this.

I realized that when exposing process_modification (without underscore), invoking this method manually does not take the influence into account, since this logic is closely coupled to the Skeleton3D design. I tried to explain this in the documentation. But I also realize that manual processing isn't really part of the design and I can imagine we might change it in the future to handle influence differently, so I decided to mark it experimental. What do you think about this function?

[TokageItLab]

In past designs, I would have considered exposing it as an advance(), but that may be a bit dangerous in the current design.

If you want to add it, I recommend to add the signal at the beginning of the process that iterates the modifier during the SKELETON_UPDATE process and return an error so that it is not fired until during that process. This is because to make blending and restoring current pose working needs that; In other words, if it is executed outside of the expected steps, the result of set_bone_pose() updated at an unintended time will remain which may cause corrupting modification calculation, moreover the Influence property will be ignored.

[TokageItLab]

LGTM from point of view of implementation

[akien-mga]

Thanks!