add presets/bitcoind-remote.nix

This simplifies integrating a remote bitcoind instance and
makes `bitcoin-cli` work with the remote node.

Add note regarding `whitelistedPort` to docs.

modules/presets/bitcoind-remote.nix

@@ -0,0 +1,25 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.services.bitcoind;
+  secretsDir = config.nix-bitcoin.secretsDir;
+in {
+  services.bitcoind = {
+    # Make the local bitcoin-cli work with the remote node
+    extraConfig = ''
+      rpcuser=${cfg.rpc.users.privileged.name}
+    '';
+  };
+
+  systemd.services.bitcoind = {
+    preStart = lib.mkAfter ''
+      echo "rpcpassword=$(cat ${secretsDir}/bitcoin-rpcpassword-privileged)" >> '${cfg.dataDir}'/bitcoin.conf
+    '';
+    postStart = lib.mkForce "";
+    serviceConfig = {
+      Type = lib.mkForce "oneshot";
+      ExecStart = lib.mkForce "${pkgs.coreutils}/bin/true";
+      RemainAfterExit = true;
+    };
+  };
+}