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.
Migrating generated files from the facts subsystem to the vars subsystem is now possible.
HowTo:
1. declare `clan.core.vars.generators.<generator>.migrateFact = my_service` where `my_service` refers to a service from `clan.core.facts.services`
2. run `clan vers generate your_machine` or `clan machines update your_machine`
Vars will only be migrated for a generator if:
1. The facts service specified via `migrateFact` does exist
2. None of the vars to generate exist yet
3. All public var names exist in the public facts store
4. All secret var names exist in the secret fact store
If the migration is deemed possible, the generator script will not be executed. Instead the files from the public or secret facts store are read and stored into the corresponding vars store
