logging: Move Audit Logs to AuditObject

[mcpherrinm]

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

As a first step, we'd like Boulder to move most (if not all) audit logs to use log.AuditObject. We'd like to target Audit logs first because we retain them for the longest period of time, and they're generally the most valuable logs for us (along with WFE logs, which are already reliably structured).

In general, the goal is to migrate towards a data structure that looks like slog or otel's logs: A fixed string message, and {key string, value any} attributes annotating that. The AuditObject interface doesn't guarantee the object is a json object (ie, a struct or map[string]any), but it seems like that's true today, and would be good to keep that true.