allow writing to timestamp-millis instead of timestamp-micros format

[admackin]

Currently there's no way that I can tell that a pydantic date-time field can be converted to a timestamp-millis field, which is the required timestamp format at my workplace. I can see here that it's hardcoded to timestamp-micros, but I also note that in the other direction, both millis and micros are converted to datetime.

I am not sure how to fix in a backwards-compatible way without requiring some kind of config setup which doesn't currently exist but if anyone has ideas I would gladly submit a PR.