DEV: add a plugin modifier to change hidden site settings (#24160)

Plugins can use a new modifier to change which site settings are hidden using the :hidden_site_settings modifier. For example:

```
register_modifier(:hidden_site_settings) do |hidden|
  (hidden + [:invite_only, :login_required]).uniq
end
```

spec/lib/site_settings/hidden_provider_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+RSpec.describe SiteSettings::HiddenProvider do
+  let(:provider_local) { SiteSettings::LocalProcessProvider.new }
+  let(:settings) { new_settings(provider_local) }
+  let(:hidden_provider) { SiteSettings::HiddenProvider.new }
+
+  describe "all" do
+    after { DiscoursePluginRegistry.clear_modifiers! }
+
+    it "can return defaults" do
+      hidden_provider.add_hidden(:secret_setting)
+      hidden_provider.add_hidden(:internal_thing)
+      expect(hidden_provider.all).to contain_exactly(:secret_setting, :internal_thing)
+    end
+
+    it "can return results from modifiers" do
+      hidden_provider.add_hidden(:secret_setting)
+      plugin = Plugin::Instance.new
+      plugin.register_modifier(:hidden_site_settings) { |defaults| defaults + [:other_setting] }
+      expect(hidden_provider.all).to contain_exactly(:secret_setting, :other_setting)
+    end
+  end
+end