feat: support custom json parsers for CloudLoggingHandler

[daniel-sanche]

We recently had a PR that adds custom JSON parser support for the StructuredLogHandler. No such feature exists for CloudLoggingHandler, which expects JSON-serializable objects only. It may be more user-friendly if both handlers supported this feature, and it could be configured as part of setup_logging

Related: googleapis/python-logging#656

[parthea]

Transferring to google-cloud-python as the code for python-logging will be migrated there in 1-3 weeks.