signed_config should use this implementation

[vanlooverenkoen]

We should update this package to signed_json_cli so we can use this as a cli and the correct implementation is done in signed_json so we don't have to maintain 2 codebases that are doing roughly the same thing.

For now they work just fine. but this is something that should be refactorred in the future.

jeroentrappers/sign_config#2