secrets: allow extending generate-secrets

`generate-secrets` is no longer a monolithic script. Instead, it's
composed of the values of option `nix-bitcoin.generateSecretsCmds`.

This has the following advantages:
- generate-secrets is now extensible by users
- Only secrets of enabled services are generated
- RPC IPs in the `lnd` and `loop` certs are no longer hardcoded.

Secrets are no longer automatically generated when entering nix-shell.
Instead, they are generated before deployment (via `krops-deploy`)
because secrets generation is now dependant on the node configuration.

modules/spark-wallet.nix

@@ -83,6 +83,13 @@ in {
       } // nbLib.allowedIPAddresses cfg.enforceTor
         // nbLib.nodejs;
     };
+
     nix-bitcoin.secrets.spark-wallet-login.user = cfg.user;
+    nix-bitcoin.generateSecretsCmds.spark-wallet = ''
+      makePasswordSecret spark-wallet-password
+      if [[ spark-wallet-password -nt spark-wallet-login ]]; then
+         echo "login=spark-wallet:$(cat spark-wallet-password)" > spark-wallet-login
+      fi
+    '';
   };
 }