Storing secrets definition in flake.nix

[hauleth]

This could help with integrating few things. Currently I have single keys.json file that describes all keys used in my system (it is managed by my fuk tool). I use the same definition of keys for nixosConfiguration as well as for secrets.nix. It would be really helpful if it would be possible to have them all managed from flake.nix as it would allow me to inject some data and helpful function into secrets.nix which is currently not possible without using stateful import <foo>.