perf: Add BulkNullStringArrayBuilder trait, use in repeat

[neilconway]

Add a DataFusion-side trait that abstracts over the bulk-NULL string array builders (GenericStringArrayBuilder and StringViewArrayBuilder), so that functions which dispatch over Utf8/LargeUtf8/Utf8View can adopt the new builders without giving up their single-bodied generic implementation.

Convert repeat as the first call site. The output is null iff either input is null, so the per-row null match becomes a single NullBuffer::union over the input null buffers, evaluated once before the loop.

Also mark the inherent append_value/append_placeholder methods on the new builders as #[inline]; without this, calls through the trait wrapper end up going through a non-inlined inherent and slow down small-output paths.

Which issue does this PR close?

• Closes Optimize repeat to use bulk-NULL string builder #21853.

Rationale for this change

Optimize NULL handling in repeat using the bulk-NULL string builders that have recently been added. This requires adding BulkNullStringArrayBuilder, a trait that is similar in spirit to Arrow's StringLikeArrayBuilder.

Benchmarks:

• repeat_string overflow [size=1024, repeat_times=1073741824]: 1022.5ns → 1054.5ns (+3.13%)
• repeat_string overflow [size=4096, repeat_times=1073741824]: 1016.6ns → 1055.3ns (+3.81%)
• repeat_large_string [size=1024, repeat_times=3]: 32.4µs → 26.6µs (−17.90%)
• repeat_large_string [size=4096, repeat_times=3]: 127.4µs → 104.0µs (−18.37%)
• repeat_string [size=1024, repeat_times=3]: 32.6µs → 26.8µs (−17.79%)
• repeat_string [size=4096, repeat_times=3]: 127.4µs → 105.5µs (−17.19%)
• repeat_string_view [size=1024, repeat_times=3]: 37.3µs → 31.7µs (−15.01%)
• repeat_string_view [size=4096, repeat_times=3]: 146.5µs → 124.5µs (−15.02%)
• repeat_large_string [size=1024, repeat_times=30]: 82.0µs → 80.4µs (−1.95%)
• repeat_large_string [size=4096, repeat_times=30]: 344.2µs → 338.7µs (−1.60%)
• repeat_string [size=1024, repeat_times=30]: 81.7µs → 79.7µs (−2.45%)
• repeat_string [size=4096, repeat_times=30]: 352.2µs → 334.7µs (−4.97%)
• repeat_string_view [size=1024, repeat_times=30]: 88.1µs → 83.1µs (−5.68%)
• repeat_string_view [size=4096, repeat_times=30]: 368.8µs → 342.6µs (−7.10%)
• repeat/scalar_utf8: 174.7ns → 179.2ns (+2.58%)
• repeat/scalar_utf8view: 174.5ns → 180.5ns (+3.44%)

What changes are included in this PR?

• Add BulkNullStringArrayBuilder
• Optimize repeat using BulkNullStringArrayBuilder
• Inline some functions in GenericStringBuilder; benchmarking suggests this is a win

Are these changes tested?

Yes.

Are there any user-facing changes?

No.