Draft: #645 added run_at_time_zone field to chain and add_job

[gaslitbytech]

Added new run_at_time_zone field to the chain table to allow for optional separate time zone per chain. The default of current_setting('timezone)` will make this change backwards compatible.

Todo:

• add unit tests. I haven't figured out how to run the unit tests locally yet.
• test migrations.
• add docs

[pashagolub]

This will never pass https://wiki.postgresql.org/wiki/Don%27t_Do_This#Don.27t_use_timestamp_.28without_time_zone.29

[pashagolub]

pg_timetable can be run in a specific timezone to solve this. See #645