[Variant] Add `Vairant::as_u*`

[klion26]

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

After implementing the Variant::as_f16, found that we have From<u*> for Variant<'_,'_>, but don't have the Varant::as_u* functions; we need to implement these functions.

Describe the solution you'd like

Add as_u* functions in parquet-varaint/src/variant.rs to conver Variant to u8/u16/u32/u64

Describe alternatives you've considered

Additional context

[klion26]

take