[FEA] Define DataFrame.__round__ for compat with round built-in function

[mroeschke]

Is your feature request related to a problem? Please describe.

In [1]: import cudf

In [3]: round(cudf.DataFrame({"col1": [1.123, 2.123, 3.123], "col2": [1.234, 2.234, 3.234]}))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[3], line 1
----> 1 round(cudf.DataFrame({"col1": [1.123, 2.123, 3.123], "col2": [1.234, 2.234, 3.234]}))

TypeError: type DataFrame doesn't define __round__ method

In [4]: import pandas as pd

In [5]: round(pd.DataFrame({"col1": [1.123, 2.123, 3.123], "col2": [1.234, 2.234, 3.234]}))
Out[5]: 
   col1  col2
0   1.0   1.0
1   2.0   2.0
2   3.0   3.0

Describe the solution you'd like
I suppose __round__ could dispatch to DataFrame.round

Describe alternatives you've considered
Just call DataFrame.round

Additional context
Add any other context, code examples, or references to existing implementations about the feature request here.

[bdice]

Related: #10177