secrets: add option 'generateSecrets'

Move this feature from a module preset to a regular option, so that it's
easily discoverable and accessible.

Simplify the implementation of `generateSecrets` by adding it to the
existing `setup-secrets` service script.

Also rename option setup-secrets -> setupSecrets.

examples/deploy-container.sh

@@ -83,8 +83,8 @@ read -d '' src <<EOF || true
     config = { pkgs, config, lib, ... }: {
       imports = [
         <${configuration}>
-        <nix-bitcoin/modules/secrets/generate-secrets.nix>
       ];
+      nix-bitcoin.generateSecrets = true;
     };
   };
 }

examples/deploy-qemu-vm.sh

@@ -26,8 +26,8 @@ nix-build --out-link $tmpDir/vm - <<'EOF'
     imports = [
       <configuration.nix>
       <qemu-vm/vm-config.nix>
-      <nix-bitcoin/modules/secrets/generate-secrets.nix>
     ];
+    nix-bitcoin.generateSecrets = true;
   };
 }).vm
 EOF

examples/minimal-configuration.nix

@@ -1,9 +1,10 @@
 { config, pkgs, lib, ... }: {
   imports = [
     <nix-bitcoin/modules/nix-bitcoin.nix>
-    <nix-bitcoin/modules/secrets/generate-secrets.nix>
   ];
 
+  nix-bitcoin.generateSecrets = true;
+
   services.bitcoind.enable = true;
   services.clightning.enable = true;