Support "first" aggregation function

[dominikgrewe]

The "first" aggregation function is currently not supported on groupby objects:

ValueError: Unrecognize aggregate function: first

I can see in the code that the function exists here: https://github.com/googleapis/python-bigquery-dataframes/blob/main/bigframes/operations/aggregations.py#L602

But it's not included in the look-up table here: https://github.com/googleapis/python-bigquery-dataframes/blob/main/bigframes/operations/aggregations.py#L606

Is there a reason not to include it?

[TrevorBergeron]

Thanks for reporting this issue. I think it should be straightforward to add support to DataFrameGroupby and SeriesGroupby. Working on a PR.