logging: Switch to slog

Let's Encrypt is working on getting more structured logs.

We'd like to have Boulder support optionally emitting JSON-structured logs. Each log message should be a JSON object, newline-delimited. The slog JSONHandler would be the typical example of what we're thinking.

We would like this to be a setting in cmd.SyslogConfig.
Alongside StdoutLevel, we'd have `StdoutFormat: "json".
We don't need this for syslog, but if it's helpful to keep things symmetric, that's fine too.

In production we currently have Boulder use Syslog for most output, so the way we'll migrate is by enabling JSON to stdout, with a new log pipeline configured that will parse the full line as JSON, instead of just the JSON={...} portion today.