Add encryption capabilities to hash storage

Problem/Motivation

Plugins should be able to provide an encryption status (and key if enabled) to encrypt hash data storage. Since we show the form protection key on the authorization form, an attacker with access to the tempstore could technically find the rest of the keys. This is mostly for applications that use a separate database (Redis, MongoDB, etc) for tempstore data. In a default installation, if an attacker has access to the tempstore, they have access to compromise pretty much anything they want anyway. Encrypting tempstore data should mitigate risk if the external tempstore database were to be compromised.