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ARROW-10959: [C++] Add scalar string join kernel

@jorisvandenbossche I've implemented this kernel as a binary (arity) kernel, so the input list array *and* the separator input string array can both be an array (see python test).

I did not implement the case where the input list is a scalar, and the separator an array, since I don't think that's very common.

And note that the kernel is named `binary_join` because it takes string-like and binary-like inputs.

Closes #8990 from maartenbreddels/ARROW-10959

Lead-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Maarten A. Breddels <maartenbreddels@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>