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

