libnm-util: add secret flags for each secret describing how the secret is stored
This allows the necessary flexibility when handling secrets; otherwise it wouldn't be known when NM should save secrets returned from agents to backing storage, or when the agents should store the secrets. We can't simply use lack of a secret in persistent storage as the indicator of this, as (for example) when creating a new connection without secrets the storage method would be abmiguous. At the same time, fold in "always ask" functionality for OTP tokens so user agents don't have to store that attribute themselves out-of-band.
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