[FEATURE-REQUEST] Aggregate dict on a groupby

[vikalfc]

When #2032 is done, it would be useful to have something like this

import vaex

df = vaex.datasets.titanic()

def custom_f(x,y):
    return {"name": x,"cabin": y}

df["content"]=df.apply(custom_f,
    arguments=[df["name"], df["cabin"]])
df_pandas=df.to_pandas_df()

df_pandas.groupby(by=['pclass', 'survived']).agg({
    'content': list}).reset_index()