Generalize StringViewArrayBuilder / StringArrayBuilder, use more broadly #21539

Is your feature request related to a problem or challenge?

We have StringViewArrayBuilder and StringArrayBuilder, which are optimized versions of corresponding string builders in Arrow. However, our versions are only used in two places; we use the Arrow versions much more often, partly because our versions have a very narrow API (you can only pass a ColumnValueRef). That means we can't use our builder versions in situations where the caller has transformed the value of the column, which is pretty common, roughly in all these places:

  | `string/common.rs` (case_conversion Utf8View path) | 359-372 | `to_upper`/`to_lower` for Utf8View |
  | `unicode/initcap.rs` | 166-172, 238-244 | Non-ASCII `initcap` for Utf8/LargeUtf8 and Utf8View |
  | `unicode/reverse.rs` | 135-153 | `reverse` for all string types |
  | `unicode/translate.rs` | 225-263, 319-357 | `translate` both all-array and scalar-optimized paths |
  | `unicode/substrindex.rs` | 183-241 | `substr_index` all-array path |
  | `string/replace.rs` | 166-181, 194-209 | `replace` for Utf8View and generic string |
  | `unicode/lpad.rs` | 237, 294, 454, 510 | Various `lpad` code paths |
  | `unicode/rpad.rs` | 238, 296, 454, 511 | Various `rpad` code paths |
  | `datetime/to_char.rs` | 209-210, 252-253 | `to_char` scalar and array format paths |
  | `string/repeat.rs` | 329 | `repeat` for generic string |

If we extended the API of our builders, we could use them in all/most of these places, which would yield a nice perf win (e.g., because we'll be doing the NULL computation in bulk, not per-row). We'd need to add something like append_value(&str), write_str() / write_char() / finish_value(), and append_empty() (placeholder for NULLs).

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