How to have skeleton/armature change with shape keys? I want to have it so when I change the value of a shapekey, it also moves a bone according to the value. So for example, when I change my character meshes eye width or height, the eye bones move with it so it can still animate properly. Another example would be change the characters leg length so when it is increased the bone moves as well to maintain the correct ratios and not distort the animations for it. I should probably also mention that I plan on using this in Unreal Engine 4 and will need it to work in engine. Any info on how to accomplish this is appreciated.
For the eyes consider adding a vertex group for a eye [bone].  This should dominate any envelope influence if the [multiply] option is disabled.

Link to Blender documentation

https://docs.blender.org/manual/en/dev/rigging/armatures/bones/properties/deform.html#armature-bones-envelope

Multiply

This option controls how the two deforming methods interact, when they are both enabled. By default, when they are both active, all vertices belonging to at least one vertex group are only deformed through the vertex groups method