Implement ZFS dataset management and snapshot configuration

- Introduced a new `datasets.nix` file to define ZFS datasets and their properties, allowing for centralized management of dataset configurations.
- Updated `default.nix` to import and enable the `zfs_manager` service, integrating it into the system configuration.
- Refactored `zfs.sh` to remove dataset creation commands, delegating dataset management to the `zfs_manager` service.
- Removed the legacy `snapshot_config.toml` file, as snapshot configurations are now handled within `datasets.nix`.
- Modified `vars.nix` to derive mountpoint paths from `datasets.nix`, ensuring consistency across the configuration.
- Created a new `zfs.nix` file to define the `zfs_manager` service and its dependencies.
- Added comprehensive tests for the `zfs_manager` functionality, covering dataset creation, property management, and error handling.

systems/jeeves/zfs.nix

@@ -0,0 +1,20 @@
+{ inputs, ... }:
+let
+  vars = import ./vars.nix;
+  jeeves_zfs = import ./datasets.nix;
+in
+{
+  services = {
+    zfs_manager = {
+      enable = true;
+      PYTHONPATH = "${inputs.self}/";
+      EnvironmentFile = "${vars.secrets}/services/snapshot_manager";
+
+      inherit (jeeves_zfs) datasets defaultSnapshots;
+    };
+
+    # Its retention config is generated from ./datasets.nix by
+    # common/optional/zfs_manager.nix, so only the credentials are set here.
+    snapshot_manager.EnvironmentFile = "${vars.secrets}/services/snapshot_manager";
+  };
+}