[FEA] Agg groupby.filter support

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

While porting some existing code from Pandas to cuDF, I have just noticed that `groupby.filter` method is not supported.

The code I am porting is the following:

> import pandas as pd
> df1 = pd.read_csv('./data/ml-20m/ratings.csv')
> df1 = df1.groupby(USER_COLUMN).filter(lambda x: len(x) >= MIN_RATINGS)

I have already found a workaround for it, but I think it would be great to add support to `groupby.filter` method.

**Describe the solution you'd like**
Having `groupby.filter` method available in cudf.

Thanks!

Miguel