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/validators/summarization_validator.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class SummarizationValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    strategy = Summarization::Base.find_strategy(val)
+
+    return true unless strategy
+
+    strategy.correctly_configured?.tap { |is_valid| @strategy = strategy unless is_valid }
+  end
+
+  def error_message
+    @strategy.configuration_hint
+  end
+end