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/lightning-loop.nix

@@ -112,5 +112,8 @@ in {
        loop-key.user = lnd.user;
        loop-cert.user = lnd.user;
      };
+     nix-bitcoin.generateSecretsCmds.lightning-loop = ''
+       makeCert loop '${optionalString (cfg.rpcAddress != "localhost") "IP:${cfg.rpcAddress}"}'
+    '';
   };
 }