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/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;