Expose the ability to choose a Dense UnionArray

[v1gnesh]

I see there's some plumbing for Dense UnionArray, but narrow currently defaults to Sparse UnionArray.
Where enums have dozens to hundreds of variants, Sparse, unlike the name suggests, "asks" for all variants' child arrays to be filled up, which is currently being done via Default.

In such cases, the ability to ask for a dense UnionArray via an attribute macro will be handy :)

[v1gnesh]

Can probably be closed as #347 is now in.