perf: Add bulk NULL-aware string builders, use in lower and upper

[neilconway]

Which issue does this PR close?

• Part of Cleanup / overhaul StringViewArrayBuilder and related types #21684

Rationale for this change

Introduce three new string array builders with bulk null tracking:

• StringArrayBuilder (Utf8)
• LargeStringArrayBuilder (LargeUtf8)
• StringViewArrayBuilder (Utf8View)

Each builder has the following API:

• append_value(&str) -- add a non-NULL value (row)
• append_placeholder() -- add a NULL row placeholder
• finish(Option) -- finish the build, specify NULLs

These are the counterpart of Arrow's GenericStringBuilder / StringViewBuilder but it
skips per-row NULL buffer maintenance, which lets callers compute the NULL buffer in
bulk when possible.

This PR also switches case_conversion to use the new APIs, which is used to
implement lower, upper, and the Spark equivalents. This improves lower / upper
performance by 3-15% on microbenchmarks. More UDFs (~10) will be converted to use
this API in future PRs.

What changes are included in this PR?

• Add new builders
• Add unit tests
• Adopt builders in case_conversion

Are these changes tested?

Yes.

Are there any user-facing changes?

No.

[EeshanBembi]

Nice builder abstraction — the null-deferred pattern is clean and removes the per-row null-buffer write overhead. A few non-blocking observations below.

[EeshanBembi]

Nit: consider exposing an append_null() alias alongside append_placeholder(). Arrow's own builders all use append_null(), and downstream contributors reaching for this builder will naturally look for that name first. Both names can coexist.

[neilconway]

Thanks for the review @EeshanBembi !

Nit: consider exposing an append_null() alias alongside append_placeholder().

I don't agree with adding an alias; I'd prefer to pick one name or the other. append_null is what StringBuilder in Arrow does, but the semantics are a little different there -- in that case, append_null is all you need to do to append a null value. In this case, calling append_placeholder isn't sufficient to add a NULL row; you also need to pass a NULL bitmap with the corresponding bit set. I think using a name other than append_null makes this a bit more clear.