Support Elixir built in Duration struct

[josevalim]

It will have loss of precision similar to other Calendar types when converted back but one can always cast to integer before/after for full precision. We should also raise if the duration has month or year set.

We need to wait until v1.17.

[billylanchantin]

@josevalim This would be great!

I was actually looking into a similar thing the other day. Polars supports calendar math:

• https://docs.pola.rs/py-polars/html/reference/expressions/api/polars.Expr.dt.offset_by.html

This differs from pl.col("foo") + timedelta in that it can take months and leap years into account.

Our %Explorer.Duration{} is equivalent to Python's timedelta. We may want to consider adding offset_by support when we add Elixir's Duration if it supports calendar math too.