Bring back bone custom poses

[BR14Nx]

Describe the project you are working on

Souls like game where character neck and back slightly bends into turning direction to have a more dynamic animation.

Describe the problem or limitation you are having in your project

I am refering to my problem described here. There is no more functionality to set a custom pose of a Skeleton3D even though the documentation says : "The overall transform of a bone with respect to the skeleton is determined by the following hierarchical order: rest pose, CUSTOM pose and pose."

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Possibility to set, get and delete a custom pose. The global pose override is not setting a custom pose, but instead overriding the normal pose which one might not want to change.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Refer to Godot 3.0

If this enhancement will not be used often, can it be worked around with a few lines of script?

Global pose override is also setting a "different pose" but not a custom pose.

Is there a reason why this should be core and not an add-on in the asset library?

Legacy support for 3.0 Skeleton code and easier to use then custom_pose_override

[Calinou]

cc @TokageItLab

[TokageItLab]

The custom pose was removed in Animation data rework for 4.0, it is never happen to come back.

If there is a custom pose, the user will need to manage the applying order of the multiple custom poses, and that effort will be the same even if the custom pose is not used and the normal bone pose is used.

To make the management of these poses and the handling of pose interrupts easier to use, we are planning to use the ModificationStack, but these improvements have not yet been completed. See also godotengine/godot#63588 (it has been closed, but I think we need to work on it again in some way). cc @fire @lyuma

[Zireael07]

@TokageItLab In this case the docs need to be updated asap to not mislead users into thinking custom poses are posslble/supported

[TokageItLab]

@Zireael07 Ah, you are right, those are out of date and it is a problem. I will send you a PR later.