Refactor approximations into abstract types

I think we can share some approximation logic by introducing abstract types like SeparableMethod, DisaggregationMethod (if that seems correct for NMDT), SegmentsSomething for sawtooth/SOS2. This would make adding more methods easier, but I don't know if we plan on adding more methods or even keeping all the methods we're using now.

I think we could also make Epigraph more like the other configurations by not having it build a whole series of epigraph cuts, which would allow the separable methods to use it, and then we can introduce a unique helper that builds a series of epigraph cuts, which I think makes more sense because right now Epigraph sorta sits outside the API I feel.

Start by locating the existing approximation method implementations and the Epigraph configuration. Compare their shared behavior, including separable methods, NMDT, sawtooth, and SOS2, before deciding whether the proposed abstract types fit. Done would require an agreed API design and a refactor that preserves the current methods and clarifies epigraph-cut construction.