Wrapper function/type for creating Scalars

[bjchambers]

          Could add a helper for creating the static somewhere, seen it in a few instances now

Originally posted by @jordanrfrazier in #784 (comment)

[bjchambers]

Arrow recently added Scalar which simplifies many of the APIs handling scalar values. We could potentially use this to replace our ScalarValue enum.

See https://github.com/apache/arrow-rs/pull/4793/files for some notes:

1. We can use Int64Array::new_scalar(...) to simplify creating a scalar of a specific type.
2. We can create a Scalar<ArrayRef> for using an array as a scalar.

This should let us create something like:

struct ScalarValue(Scalar<ArrayRef>);

// 1. Serde the underlying ArrayRef
// 2. Support use as a scalar anywhere

[bjchambers]

@jordanrfrazier FYI -- I think we should be able to completely replace our ScalarValue enum with Scalar<ArrayRef>. I think it's probably worth doing that for how we serialize scalars in the physical plan.

Thoughts on:

A) Trying to change the ScalarValue to use that representation (potentially messy since we have that protobuf in many places0.
B) Introducing a parallel Scalar??? to represent that?
C) Just using arrow_array::Scalar<arrow_array::ArrayRef> in those places?

I think with C we'd need to do the custom serialization like we did for structs with an ArrayRef, while with A or B we could encapsulate that in our serialization of the wrapper type we create.