{:datetime, :second} dtype support

[LostKobrakai]

Looking at the initial PR for the precision on datetimes I didn't find seconds as a unit being discussed. I'm dealing with a lit of unix timestamps and doing {:datetime, :millisecond} feels wrong given I initially don't have that much precision. Are there any reasons not to support second precision?

[josevalim]

If it is supported in polars, I believe we can totally support it here too.

[LostKobrakai]

Ah, that's where the limitation is. It only supports the 3 precisions we have it seems. I was looking at the arrow docs, which were linked on #867 and it showed second as a unit.