[Feature?] Use from devShell flake with direnv

[ppenguin]

Since activating secrets is "just" shell logic (called a service and/or activation script currently), it should also be possible to have sops-nix provide a function that generates a script that can be used in a devShell env?

It could set an env var like _SOPS_SCRIPT that, when executed, populates the secrets, and/or has additional functionality to make it sourcable and set env vars with secrets defined in the dev repo directly?