feat: Evaluate x-viur-bonelist on default viewSkel()

[phorward]

• Introduces generic SkelModule.skel() function to obtain a skeleton instance.
• Modifies all prototypes to accept bones_from_request

Replacement for #1376

[sveneberth]

• *args get lost
• bones_from_request cannot be controlled by calling viewSkel()? --> Please add in viewSkels signature and pass it.

      def viewSkel(self, *args, bones_from_request : bool=True, **kwargs) -> SkeletonInstance:

Suggested change

	return self.skel(bones_from_request=True, **kwargs)
	return self.skel(*args, **kwargs)

[phorward]

• *args get lost

I want to get rid of this. It is a relic from days gone by and should be eliminated. To do it, we have to refactor all *Skel()-related stuff in prototypes, and it will also become a breaking change. Therefore, just throw it away for now here.

• bones_from_request cannot be controlled by calling viewSkel()? --> Please add in viewSkels signature and pass it.

We can do it this, do you want it to overwrite this as a default behavior?

[sveneberth]

Now we have a header that controls a bonelist. The header is evaluated by default for all viewSkels.
But what happens if I load another skeleton within the same request? Then the same bonelist is also used for the other skel. You don't want that ...

Furthermore, the Vary header should be set in the response after (and if) the bonelist has been evaluated and applied.