de / serialization of a CartesianIndex

I expected to be able to round-trip the serialization of a `CartesianIndex`:
```julia
julia> df = DataFrame(a = 1, b = CartesianIndex(1,1))
1×2 DataFrame
Row │ a b
│ Int64 Cartesia…
─────┼─────────────────────────────
1 │ 1 CartesianIndex(1, 1)

julia> Arrow.write("table.arrow", df)
"table.arrow"

julia> df = DataFrame(Arrow.Table("table.arrow"))
1×2 DataFrame
Row │ a b
│ Int64 NamedTup…
─────┼──────────────────────
1 │ 1 (I = (1, 1),)
```

Start by reproducing the shown DataFrame round trip with Arrow.write and Arrow.Table, using a CartesianIndex column. Trace how CartesianIndex is serialized and reconstructed, then verify that reading the written table returns CartesianIndex values rather than the displayed NamedTuple representation.

Written by the indexing model from the issue text.