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

[phorward]

This is a lighter version of #1384, integrating the feature only for Singleton and List.

[sveneberth]

I've in a project already a custom quick & dirty solution, based on #1384.

    def skel_from_header(
        self,
        *args,
        bones_exclude: tuple[str, ...] | t.List[str] = (),
        **kwargs,
    ) -> SkeletonInstance:
        X_VIUR_BONELIST: t.Final[str] = "X-VIUR-BONELIST"

        skel_cls = self._resolveSkelCls(**kwargs)
        bones = set()

        if bonelist := current.request.get().request.headers.get(X_VIUR_BONELIST):
            if "*" not in skel_cls.subSkels:  # a named star-subskel "*"" must exist!
                raise errors.BadRequest(f"Use of {X_VIUR_BONELIST!r} requires for a star-subskel")
            bones |= {bone.strip() for bone in bonelist.split(",")}

        bones.difference_update(bones_exclude)

        if bones:  # Return a subskel?
            log.debug(f"{current.request.get().response.vary=}")
            current.request.get().response.vary = (X_VIUR_BONELIST, *(current.request.get().response.vary or ()))
            # When coming from outside of a request, "*" is always involved.
            return skel_cls.subskel("*", bones=bones)

        elif bones_exclude:  # Return full skel, without generally excluded bones
            bones.update(skel_cls.__boneMap__.keys())
            bones.difference_update(bones_exclude)
            return skel_cls(bones=bones)

        # Otherwise, return full skeleton
        return skel_cls()

Can you please integrate the bones_exclude param and the vary header in your method too?

[sveneberth]

Why don't you implement this directly in baseSkel? Now you have to customize every call (baseSkel() --> skel()). If you would implement this in baseSkel, all calls could remain the same and you would be more backwards compatible.

With yet another *skel method, it only gets more complicated.