[Feature request] aggregate syntax and quantile computation

Hi,

The Dask API provides a method to compute quantiles of Series:
https://docs.dask.org/en/latest/dataframe-api.html#dask.dataframe.DataFrame.quantile

This is already a great thing as quantile distributed computation is known to be a real challenge.

Unfortunately, this is not available using aggregate API.
https://docs.dask.org/en/latest/dataframe-groupby.html#aggregate

Would it be possible to provide quantile computation to Dask aggregate syntax?