Support for JSON log format

[wavermartijn]

When running karapace in a container on a platform that includes centralized logging, it would be helpful to be able to output the logs formatted as JSON. I tried doing this with the KARAPACE_LOG_FORMAT option. This works for most of the log lines, but some log lines include double quotes and these will then corrupt the outputted JSON.

Native JSON output or at least a way to use the log_format to generate valid JSON would help a lot to be able to ingest the logs.