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ARROW-9254: [C++] Split out CastNumberToNumberUnsafe function from scalar_cast_numeric, add data()/mutable_data() functions for accessing primitive scalar data opaquely

This is some preparatory work for ARROW-9196. I also addressed some prior uncleanliness related to unboxing temporal scalars based on C types. By adding these `data()` and `mutable_data()` functions we can obtain a pointer e.g. to the `int64_t` stored in the scalar. Previously I was resorting to some slightly hacky inheritance tricks -- this seems better.

Closes #7561 from wesm/ARROW-9254

Authored-by: Wes McKinney <wesm@apache.org>
Signed-off-by: Wes McKinney <wesm@apache.org>