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
```

lib/site_settings/hidden_provider.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module SiteSettings
+end
+
+# A class to store and modify hidden site settings
+class SiteSettings::HiddenProvider
+  def initialize
+    @hidden_settings = Set.new
+  end
+
+  def add_hidden(site_setting_name)
+    @hidden_settings << site_setting_name
+  end
+
+  def all
+    DiscoursePluginRegistry.apply_modifier(:hidden_site_settings, @hidden_settings)
+  end
+end