[FEA] Implement DatetimeProperties.date

[charlesbluca]

Is your feature request related to a problem? Please describe.
In Pandas, it is possible to extract the date from a column of datetime objects using the date attribute of the DatetimeProperties:

import cudf

gsr = cudf.Series(cudf.date_range("2000", freq='s', periods=6))
psr = gsr.to_pandas()

psr.dt.date
0    2000-01-01
1    2000-01-01
2    2000-01-01
3    2000-01-01
4    2000-01-01
5    2000-01-01
dtype: object

This attribute doesn't exist in cuDF's DatetimeProperties:

gsr.dt.date
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-16-53087fa7f6a3> in <module>
----> 1 gsr.dt.date

AttributeError: 'DatetimeProperties' object has no attribute 'date'

Describe the solution you'd like
It would be nice to have a date attribute for the DatetimeProperties to match up with the one in Pandas.

Describe alternatives you've considered
It is possible to compute roughly the same series using strftime:

gsr.dt.strftime("%Y-%m-%d")
0    2000-01-01
1    2000-01-01
2    2000-01-01
3    2000-01-01
4    2000-01-01
5    2000-01-01
dtype: object

I'm not sure if there's a more performant way to achieve this using _get_dt_field.

Additional context
This is coming up as a blocker for some datetime casting work in dask-contrib/dask-sql#343. If it is reasonably performant to use strftime here we can use that instead of date.

[beckernick]

I believe this is a duplicate of #7880

@charlesbluca can we consolidate discussion in that issue?

[charlesbluca]

Sure!