[Variant] Reverse VariantAsPrimitive trait to PrimitiveFromVariant

[scovich]

Which issue does this PR close?

• Relates to [Variant] Add variant to arrow primitive support for boolean/timestamp/time #8515
• Relates to [Variant] Add variant to arrow primitive support for boolean/timestamp/time #8516

Rationale for this change

The existing VariantAsPrimitive trait is kind of "backward" and requires very complex type bounds to use, e.g.:

impl<'a, T> VariantToPrimitiveArrowRowBuilder<'a, T>
where
    T: ArrowPrimitiveType,
    for<'m, 'v> Variant<'m, 'v>: VariantAsPrimitive<T>,

This is a code smell.

What changes are included in this PR?

"Reverse" the trait -- instead of extending Variant, the trait extends T: ArrowPrimitiveType. The resulting type bounds are much more intuitive:

impl<'a, T: PrimitiveFromVariant> VariantToPrimitiveArrowRowBuilder<'a, T>

Are these changes tested?

Existing unit tests cover this refactor.

Are there any user-facing changes?

No.

[scovich]

CC @klion26

[alamb]

looks good to me too -- thank you @scovich and @mbrobbel