[C++] Optimize ArrayBuilder::AppendScalar

[asfimport]

Hmm, it looks like the implementation could be more optimized. When appending multiple scalars (or multiple times the same scalars), the required space could be reserved. Also, at least for primitive types, I would expect appending multiple scalars to be much more streamlined than this.

(it looks like such optimizations could be crafted more easily if AppendScalar was a bunch of virtual methods?)

From #10579 (comment)

Also, dictionary scalars aren't supported. You could imagine both appending a dictionary scalar to a regular builder (decoding the value) and appending a dictionary or regular scalar to a dictionary builder (encoding the value as necessary).

Reporter: David Li / @lidavidm

Related issues:

• [C++] Provide ArrayBuilder::AppendScalar (relates to)

_{Note: This issue was originally created as ARROW-13197. Please see the migration documentation for further details.}