This adds options `invalidationData` to generators.
`invalidationData` can be used by an author of a generator to signal if a re-generation is required after updating the logic.
Whenever a generator with invalidation data is executed, a hash of that data is stored by the respective public and/or secret backends.
The stored hashes will be checked on future deployments, and a re-generation is triggered whenever a hash doesn't match what's defined in nix.
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