Support JSON type for Secrets

[heni-nechi]

Hey! I wanted to toss an idea out there for handling JSON secrets. Since providers like AWS Secrets Manager usually bundle credentials (user, pass, host) into a single JSON object, it would be super helpful if fnox could parse that natively. Instead of getting one big stringified blob that the app has to decode, maybe we could have a type = "json" option? Ideally, this could either let us extract specific keys (like key="username") or even auto-expand the whole object into separate environment variables. It’d make syncing complex secrets way smoother.
Thoughts?