[Feature request] Optional secrets #665

Engine fails closed if it can't resolve the secret, so it's up to a caller to provide a resolvable reference upfront.

Would be nice to be able to specify that the secret ref is optional to resolve - like se(+optional;empty)://some/ref which will be resolved by empty value if it can't be resolved for some reason - missing or auth declined.

The usecase:

We have a tool that lists infrastructure components and collects all info in a single dashboard. Sometimes token expires and have to be reauthorized manually and updated in the engine. However, when tool queries engine and the engine fails closed, we have to restart the flow excluding the secret completely, instead of just skipping it, as we can safely ignore part of the dashboard for some time.