perf: Optimize NULL handling in StringViewArrayBuilder

[neilconway]

Which issue does this PR close?

• Closes Optimize NULL handling in StringViewArrayBuilder #21537.

Rationale for this change

StringViewArrayBuilder is implemented on top of Arrow's StringViewBuilder; the latter tracks NULLs incrementally. However, the StringViewArrayBuilder requires callers to pass a NULL buffer to finish() anyway, so the NULL bitmap that has been computed by StringViewBuilder is discarded. It would be more efficient to stop using StringViewBuilder so that we don't do this redundant work; in theory there might be room for inconsistency between the two NULL bitmaps as well.

Right now, StringViewArrayBuilder is only used by the concat and concat_ws UDFs, but I'd like to generalize the API and use it more broadly in place of StringViewBuilder (#21539). For the time being, here are the results of this PR on the concat benchmarks (Arm64):

  - 1024 rows: 29.6 µs → 28.0 µs, -5.3%
  - 4096 rows: 134.3 µs → 125.6 µs, -6.5%
  - 8192 rows: 289.7 µs → 273.5 µs, -5.6%

What changes are included in this PR?

• Stop using StringViewBuilder and build the views ourselves
• Improve some comments

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

[neilconway]

FYI @Omega359

[Omega359]

I looked this over and it seems reasonable, thanks!