Add Postgres config support to Praefect

Part of #1755 (closed)

The idea here is that we log a message during Praefect startup that tells us whether Praefect can connect to Postgres. This also means we are adding fields to the Praefect config file for configuring a Postgres connection.

For now this is purely diagnostic; this will allow us to merge this first and then gradually add Postgres config and support to all existing Praefect installation methods and deployments.

Besides logging a message, this also adds a subcommand that lets you ping Postgres from the command line:

praefect -config /path/to/config.toml sql-ping