Tracking Issue: Add an Array variant to ScalarValue #6771

Now that we no longer have a vortex-scalar crate (it has been absorbed by vortex-array, see #6523), we are now able to add an Array variant to ScalarValue to represent scalars like list scalars much more compactly.

By passing around ArrayRef instead of Vec<Option<ScalarValue>> we can likely improve the performance of list-related operations. And in the near future where we would like to have a Tensor type, it is far more ideal to be passing around references directly to the memory rather than making a huge amount of allocations.

This will require us adding a new variant to the protobuf serialization format, as well as passing VortexSession around in a few more places since we now need to validate any Array scalar value variant on deserialization.