Adding JSON Marshalling to Config

[praetoriansentry]

This change is adding a toJson() method to the Config class and using the Json representation of the Config in order to print.

At a high-level, printing the config in Json format has a few potential advantages:

• It's a bit easier to verify the loaded configuration against the config file since they're both machine readable
• It's a nice feature to be able to dump the default config.
  • echo "{}" > empty.json; ./build/zkProver -c empty.json will now print out the defaults that can be copy pasted into a new file and the tweaked
• I'm hopeful that this is easier to maintain going forward because the toJson() method is just a copy paste / macro of the load(json &config) method
• The method itself could be useful in other future scenarios where the config needs to be serialized for some reason

[cla-bot]

We require contributors/corporates @praetoriansentry to read our Contributor License Agreement, please check the Individual CLA document/Corporate CLA document