Allow the zip compute function to operator on Scalar values via Datum

[Nathan-Fenner]

Which issue does this PR close?

Closes #5011.

Rationale for this change

The arrow compute kernel function zip can now work on scalars, allowing a scalar value to be "zipped" with another array, repeating the scalar where needed to fill in the selected spots:

Before	After
pub fn zip( mask: &BooleanArray, truthy: &dyn Array, falsy: &dyn Array, ) -> Result { ... }	pub fn zip( mask: &BooleanArray, truthy: impl Datum, falsy: impl Datum, ) -> Result { ... }

If a Scalar value is provided, then it will be copied to fill every spot specified in the mask. Previously, callers would be required to implement this logic internally to support this.

Since &dyn Array already implements Datum, existing calls will still continue to work exactly as they did before, but it is now also possible to pass in a scalar value.

In addition, &dyn Datum now implements Datum. This means that zip can be called with the same signature as e.g. add, which explicitly ask for &dyn Datum; otherwise, it would not be possible to call zip with the same kind of signature as add.

(I think in a future PR, it would be a good idea to change add, sub, and all of the other computing kernels to take impl Datum parameters instead of &dyn Datum parameters for maximum flexibility and efficiency)

What changes are included in this PR?

• zip's signature changes from &dyn Array to impl Datum
• &dyn Datum now implements Datum

Are there any user-facing changes?

• zip now supports Scalar/Datum arguments
• &dyn Datum now implements Datum

[tustvold]

I took a quick look and this looks good thank you, I would ask that we rollback the Datum changes though - there is a reason we don't do this 😅

[tustvold]

This looks good to me, thank you

[tustvold]

This is kind of unfortunate, but there aren't really many good alternatives that occur to me. Ultimately if this kernel shows up a bottleneck, it should be relatively straightforward to special case for primitives, etc... like we do for the other selection kernels

Edit: or possibly this could just call the interleave kernel 🤔

Edit edit: filed #5097