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/btcpayserver.nix

@@ -253,5 +253,8 @@ in {
       };
       bitcoin-HMAC-btcpayserver.user = cfg.bitcoind.user;
     };
+    nix-bitcoin.generateSecretsCmds.btcpayserver = ''
+      makeBitcoinRPCPassword btcpayserver
+    '';
   };
 }