DEV: implements register_modifier(:custom_homepage_enabled) (#28657)

This commit will allow plugin developers to enable/disable the custom homepage.

Usage:

```ruby
register_modifier(:custom_homepage_enabled) do |enabled, args|
  true
end 
```

Args might contain request and/or current_user.

lib/homepage_helper.rb

@@ -4,6 +4,16 @@ class HomepageHelper
   def self.resolve(request = nil, current_user = nil)
     return "custom" if ThemeModifierHelper.new(request: request).custom_homepage
 
+    enabled = false
+    enabled =
+      DiscoursePluginRegistry.apply_modifier(
+        :custom_homepage_enabled,
+        enabled,
+        request: request,
+        current_user: current_user,
+      )
+    return "custom" if enabled
+
     current_user ? SiteSetting.homepage : SiteSetting.anonymous_homepage
   end
 end