Generate stderr logs for barretenberg with proving progress

[ludamad]

We want to print json-encoded log lines for proving events that are intended for use by something like zkpassport to show proving progress. While they are interested in a percentage, that is too high level for us. We prefer to instead have low-level JSON-encoded progress. It is fine to co-opt existing logs for this. they will be enabled with some event log flag or global. We can add a bbapi endpoint for setting bb settings like this and other flags (e.g. low memory proving)