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ARROW-7948: [Go] Decimal128 Integration fix

Had to redesign a bit of the arrjson handling to do this properly, but I think it's better this way anyways.

Closes #10116 from zeroshade/decimal_integration

Authored-by: Matthew Topol <mtopol@factset.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>