Cleanup UnionArray Constructors

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The UnionArray constructors are somewhat at odds with the pattern followed for other array types.

Describe the solution you'd like

I would like the signature to be updated to be

fn try_new(fields: UnionFields, type_ids: ScalarBuffer<i8>, offsets: Option<ScalarBuffer<i32>>, children: Vec<ArrayRef>) 
    -> Result<Self, ArrowError>

into_parts should then be updated to match

Describe alternatives you've considered

Additional context