Custom output formatter for logging

PR https://github.com/envoyproxy/nighthawk/pull/680 added more logging that is useful for a specific application of the adaptive load controller. The logging strips uninteresting parts of the NH output for brevity.

It would be interesting to implement generic logging output formatter functionality. This allow NH users to implement their own logging formatters, logging the set of information that is useful for their application.

The implementation could use a plugin based system allowing to implement various logging formatters and a command line option to choose which logging formatter to use for the execution.