Add support for 5 bone chains to the limbs.paw rig.
Implement a new limbs.rear_paw rig, which provides a three bone IK
mechanism designed to keep the first and third bones nearly parallel
by default (based on a YouTube video by @Pieriko as suggested by
@icappiello).
Implement a limbs.front_paw rig with automation that aims to
keep the angle between second and third bones mostly stable
by default (has influence option), as suitable for front paws.
The horse and wolf metarigs are updated to use these new rig
types, with the horse rig further overhauled by @icappiello.
Maniphest Tasks: T78463
Differential Revision: https://developer.blender.org/D8496
