GH-3316: Fix representation type for VariantBuilder decimal

[ArnavBalyan]

Rationale for this change

• Since VariantBuilder decimal looks at scale for deciding the representation, it can cause low precision numbers to be stored in wide format.
• Removed the scale looking just at decimal similar to SchemaConverter.visit(Decimal) in Parquet
• Added tests to prevent future regression, Closes: VariantBuilder does not choose the tightest decimal type possible #3316

Are these changes tested?

• Yes

Are there any user-facing changes?

• Yes

[ArnavBalyan]

cc @gszadovszky could you pls take a look :)