improvement(coercer transform): add a second chrono format option to convert from datetime to string

[xcaptain]

This is an attempt to fix #867 feel free to close if it's inappropriate

[binarylogic]

Thanks @xcaptain. I think the format is a little confusing given that the user is producing a string, not a timestamp. If I understand correctly, you basically want to re-format a string timestamp by converting it to a timestamp and the converting it back to a string in a different format?

[xcaptain]

Yes, the first format convert a string to datetime and the second format convert datetime to another string. I will give my case as an example:

I'm using vector to collect nginx access log and then store to clickhouse. nginx uses $time_iso8601 which produces datetime like 2019-09-16T18:17:54+08:00 but by default clickhouse only accept datetime like 2019-09-16 18:17:54, to accept iso 8601 datetime, I have to modify clickhouse settings https://clickhouse.yandex/docs/en/operations/settings/settings/#settings-date_time_input_format I think it's better to let vector do this datetime conversion

[binarylogic]

That makes sense. If you wouldn't mind, give us this week to discuss. I think there is a clearer more user-friendly approach to this. The fact that timestamp is used as the explicit type, but then produces a string is confusing. The alternative I'm considering is improving how users can transform data. Ex: coercing to a timestamp and then coercing back to a string. This is being discussed in #750.

[xcaptain]

@binarylogic Is there any methods to convert a timestamp from 2019-09-16T18:17:54+08:00 to 2019-09-16 18:17:54 currently?