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/summarization/base.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+module Summarization
+  class Base
+    def self.available_strategies
+      DiscoursePluginRegistry.summarization_strategies
+    end
+
+    def self.find_strategy(strategy_model)
+      available_strategies.detect { |s| s.model == strategy_model }
+    end
+
+    def self.selected_strategy
+      return if SiteSetting.summarization_strategy.blank?
+
+      find_strategy(SiteSetting.summarization_strategy)
+    end
+
+    def initialize(model)
+      @model = model
+    end
+
+    attr_reader :model
+
+    def can_request_summaries?(user)
+      user_group_ids = user.group_ids
+
+      SiteSetting.custom_summarization_allowed_groups_map.any? do |group_id|
+        user_group_ids.include?(group_id)
+      end
+    end
+
+    def correctly_configured?
+      raise NotImplemented
+    end
+
+    def display_name
+      raise NotImplemented
+    end
+
+    def configuration_hint
+      raise NotImplemented
+    end
+
+    def summarize(content)
+      raise NotImplemented
+    end
+  end
+end