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'

plugins/chat/config/routes.rb

@@ -35,6 +35,8 @@ Chat::Engine.routes.draw do
 
     put "/channels/:channel_id/messages/:message_id/restore" => "channel_messages#restore"
     delete "/channels/:channel_id/messages/:message_id" => "channel_messages#destroy"
+
+    get "/channels/:channel_id/summarize" => "summaries#get_summary"
   end
 
   # direct_messages_controller routes