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ARROW-15218: [C++] Add decimal support to the indices_nonzero compute function

Adds decimal support to the `indices_nonzero` compute function. This vector function returns the indices of an array that contains values `!=0` or `!=false`.

This can be used in conjunction with existing functions that return a mask to get back the indices where the mask matches.

Have also added a small enhancement to the `indices_nonzero` tests to use a `TYPED_TEST_SUITE` for numeric primitives.

Closes #12270 from c-jamie/ARROW-15218-non-zero-decimal

Lead-authored-by: c-jamie <jamie.b.clery@gmail.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>