Transparently support enum/union with 127+ variants

[v1gnesh]

When looking through Union layout in the arrow columnar format, I noticed that within Dense Union, it says, "A union with more than 127 possible types can be modeled as a union of unions."

Believe this is left as an exercise to any union array implementer, and not something that happens automagically.
A workaround for this is to split bigger enums into ones with <=127 types.
It would be nice if this can be handled transparently in narrow, if possible 🙏

Maybe an unrelated reference... DuckDB's union type supports 256 variants.