Fix timestamp conversion from Pandas to Python datetime in streaming mode
Arrow accepts both pd.Timestamp and datetime.datetime objects to create timestamp arrays.
However a timestamp array is always converted to datetime.datetime objects.

This created an inconsistency between streaming in non-streaming. e.g. the `ett` dataset outputs datetime.datetime objects in non-streaming but pd.timestamp in streaming.

I fixed this by always converting pd.Timestamp to datetime.datetime during the example encoding step.
I fixed the same issue for pd.Timedelta as well. Finally I added an extra step of conversion for Series and DataFrame to take this into account in case such data are passed as Series or DataFrame.

Fix https://github.com/huggingface/datasets/issues/4533
Related to https://github.com/huggingface/datasets-server/issues/397