[Variant] Add input validation in `VariantBuilder`

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
As @scovich points out, there are some data types such as Decimal or Binary where the Rust native type can express values that are not allowed in Variants

• Initial Builder API for Creating Variant Values #7653 (comment)
• Initial Builder API for Creating Variant Values #7653 (comment)

Describe the solution you'd like
It would be nice to ensure that the VariantBuilder could only produce valid Variant values

Describe alternatives you've considered

To do so we would probably have to add error checking to the builder functions like new_decimal 🤔

Additional context

[Weijun-H]

take

[Weijun-H]

It would be better to provide two append functions: one that includes validation and returns an error, and another without validation. 🤔

[alamb]

It would be better to provide two append functions: one that includes validation and returns an error, and another without validation. 🤔

Another approach that @friendlymatthew @scovich and I came up with in #7718 was to introduce a "newtype" wrapper that validated on construction (and this prevents invalid Variant's from being constructed)

For example, for Decimal we could introduce

enum Variant {
..
  Decimal4(VariantDecimal4)
..
}

Where VaraintDecimal4::try_new did the validation for precision /scale

That would avoid having any error path in the builder and I think be eaiser to reason about

What do you think @Weijun-H ?

[alamb]

label_issue.py automatically added labels {'parquet'} from #7776