FEATURE: Custom content summarization strategies. (#21813)

* FEATURE: Content custom summarization strategies.

This PR establishes a pattern for plugins to register alternative ways of summarizing content by extending a class that defines an interface.

Core controls which strategy we'll use and who has access to it through the `summarization_strategy` and `custom_summarization_allowed_groups`. It also defines the UI for summarizing topics.

Other plugins can access this summarization mechanism and implement their features, removing cross-plugin customizations, as it currently happens between chat and the discourse-ai plugin.

* Group membership validation and rate limiting

* Work with objects instead of classes

* Port summarization feature from discourse-ai to chat

* Rename available summaries to 'Top Replies' and 'Summary'

lib/discourse_plugin_registry.rb

@@ -113,6 +113,8 @@ class DiscoursePluginRegistry
 
   define_filtered_register :list_suggested_for_providers
 
+  define_filtered_register :summarization_strategies
+
   def self.register_auth_provider(auth_provider)
     self.auth_providers << auth_provider
   end