Support for the default datetime fmt when using .cast(pl.Datetime)

[mkleinbort-ic]

Description

Currently this code works:

df = pl.DataFrame({'datetime': ['2024-03-12 19:53:32']})

df.select(pl.col('datetime').str.to_datetime())

But this code fails

df.select(pl.col('datetime').cast(pl.Datetime))

I don't see a good argument for the .cast(pl.Datetime) not to default to just calling str.to_datetime.

That seems to be the behavior with dates.

df = pl.DataFrame({'datetime': ['2024-03-12']})

# This works
df.select(pl.col('datetime').str.to_datetime())

# This also works
df.select(pl.col('datetime').cast(pl.Date))