Altering fn save_state to save sensitive data in stronghold and the rest in json

[Oran-Dan]

Comment by Nander on the PR feat/refactor AppState, add FeatTrait
"I guess perhaps each Feature should have the ability to set its persistence strategy, either pure json file or stronghold. So pub async fn save_state(app_state: &AppState) would be pub async fn save_state(app_state: Box) or something similar.

Perhaps even better it could/should be implemented via the FeatTrait. (for another time though, I think the current solution, although not super clean, simply does the job)."