ENH: Add support for `date_unit` to be specified per column in `to_json`

[galipremsagar]

Is your feature request related to a problem?

I would want to encode my dataframe consisting of datetime64 & timedelta64 columns with different units of precision per column in to_json.

Describe the solution you'd like

Currently in to_json, the date_unit param achieves this for the entire dataframe, but I would like to encode each column with different precision. Hence a dict-like support for date_unit would be good to have.

Describe alternatives you've considered

None

[rhshadrach]

Is this for read_json?

[galipremsagar]

@rhshadrach This is in to_json, but yes change in to_json would also need changes in reader too.