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ARROW-9434: [C++] Store type code in UnionScalar

Also:
- implement MakeArrayFromScalar for unions
- implement ArrayBuilder::AppendScalar for unions

Closes #10817 from pitrou/ARROW-9434-union-scalar-type-code

Lead-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>