FEATURE: Allow site settings to be edited throughout admin UI (#26154)

This commit makes it so the site settings filter controls and
the list of settings input editors themselves can be used elsewhere
in the admin UI outside of /admin/site_settings

This allows us to provide more targeted groups of settings in different
UI areas where it makes sense to provide them, such as on plugin pages.
You could open a single page for a plugin where you can see information
about that plugin, change settings, and configure it with custom UIs
in the one place.

In future we will do this in "config areas" for other parts of the
admin UI.

lib/site_setting_extension.rb

@@ -176,7 +176,13 @@ module SiteSettingExtension
   end
 
   # Retrieve all settings
-  def all_settings(include_hidden: false)
+  def all_settings(
+    include_hidden: false,
+    include_locale_setting: true,
+    only_overridden: false,
+    filter_categories: nil,
+    filter_plugin: nil
+  )
     locale_setting_hash = {
       setting: "default_locale",
       default: SiteSettings::DefaultsProvider::DEFAULT_LOCALE,
@@ -189,12 +195,28 @@ module SiteSettingExtension
       translate_names: LocaleSiteSetting.translate_names?,
     }
 
+    include_locale_setting = false if filter_categories.present? || filter_plugin.present?
+
     defaults
       .all(default_locale)
       .reject do |setting_name, _|
         plugins[name] && !Discourse.plugins_by_name[plugins[name]].configurable?
       end
       .reject { |setting_name, _| !include_hidden && hidden_settings.include?(setting_name) }
+      .select do |setting_name, _|
+        if filter_categories && filter_categories.any?
+          filter_categories.include?(categories[setting_name])
+        else
+          true
+        end
+      end
+      .select do |setting_name, _|
+        if filter_plugin
+          plugins[setting_name] == filter_plugin
+        else
+          true
+        end
+      end
       .map do |s, v|
         type_hash = type_supervisor.type_hash(s)
         default = defaults.get(s, default_locale).to_s
@@ -222,7 +244,15 @@ module SiteSettingExtension
 
         opts
       end
-      .unshift(locale_setting_hash)
+      .select do |setting|
+        if only_overridden
+          setting[:value] != setting[:default]
+        else
+          true
+        end
+      end
+      .unshift(include_locale_setting && !only_overridden ? locale_setting_hash : nil)
+      .compact
   end
 
   def description(setting)