Allow user-specified log levels for client, service and subscription related events

[m-ronnblom]

Extend the pafd log file with the possibility to specify the log level for information related to subscribe, unsubscribe, publish, and unpublish events.

Currently, all logs related to such events (e.g., a service is published) is logged on debug level. In certain deployments, it makes sense to trade some performance/memory/CPU-cycles for observability, and have those kind of events logged, but full debug logging is a too verbose.

Currently, client connection established and tear down events are logged at info level. This should also be a matter of configuration, so that performance-optimized deployments may classify those as debug level log entries.

The default behavior should be to keep legacy log levels (i.e., debug for subscriptions/services and info level for client connection establishment/disconnect.