Support viewing `datetime` values in a specific time zone

We should support a way to set your local time zone so that when you make queries that return `datetime` values, we show them in this time zone rather than as UTC. This makes doing business analysis in the REPL way easier. Bonus points if the JSON output can be formatted to produce ISO8601 in this time zone as well.

Postgres has a session-level setting like `SET timezone to 'America/New_York';` that has this effect, and we want to provide a similar affordance.