DBZ-8689 Provide config to override all date/datetime cols to nullable

[twthorn]

Ticket https://issues.redhat.com/browse/DBZ-8689

For nullable cols, we send null when we receive a zero date. For not nullable columns, we send the epoch value. It is impossible for consumer to differentiate between true epoch and zero value. Provide a setting to override these columns to nullable so that a special value (null) can be sent when a zero date is encountered that is distinct from the epoch value.

Considered using a custom converter, but it appeared to be an overly complex solution. Since these are temporal types, they conversion logic is dependent on the temporal mode eg for date or datetime. The converter would need to read in all these configs, and duplicate the logic (since the value conversion should be the same). So code is no longer dry and have potential for drift. All we need is to set the schema to be optional (but want an identical converter). I didn't see a good way to do this (nor a good way to read in those temporal configs in a custom converter), so I opted for a separate config that is read in when building the schema & value converters inVitessValueConverter. Please let me know if there's a better way to do this.

This config seems more akin to time.precision.mode setting which could technically be done with a custom converter but not without significant downside, so instead we have the connectors' main value converter read it in and handle the conversion logic there.

[twthorn]

@jpechane this is also ready for review when you get a free moment, thanks!

[jpechane]

@twthorn Applied, thanks! Could you please send a PR with docs update too?