added pole target feature with pole angle

[ademola-lou]

added pole target feature, the pole target controls the bending direction of the ik chain making it more predictable.

[ademola-lou]

Test
https://github.com/user-attachments/assets/0c9bbf37-5d12-42aa-9107-bfadc60903eb

[japopra]

Hi, it has been some busy months, so we did not have time to review this PR until now.

After reviewing the code, the idea really seems promising, but we cannot merge these features yet:

• The poleAngle attribute feels a bit redundant. Users could simply update the poleTarget.
• It would be better if it could generalise to any chain instead of hardcoding it for three bones.
• If poleDirection is aligned with the chainAxis, maybe it could simply abort, instead of trying to find a "valid" direction.
• There is an issue with the projectedLength computation, as it assumes that the line between chain[1] and midPoint is perfectly perpendicular to chainAxis, which is not always true. Thus, at the last line, when recomputing the chain[1] position, the resulting point lies in the correct plane but in the wrong location (see attached image).

If you are available to address these concerns, we can keep talking of your contribution :)