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ARROW-12951: [C++] Reduce generated code size for string kernels

Factor out type-agnostic string operations (such as finding a split pattern) in separate classes to avoid generating several versions of them when generating the typed kernel execution classes. This also makes the code slightly easier to understand and maintain (IMHO) by reducing use of subclassing.

Also fix a bug where some kernels would error out on invalid UTF8 data, even when it's the masked payload of a null value.

Closes #10496 from pitrou/ARROW-12951-string-transform-refactor-v2

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>