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'

spec/support/dummy_custom_summarization.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+class DummyCustomSummarization < Summarization::Base
+  RESPONSE = "This is a summary of the content you gave me"
+
+  def display_name
+    "dummy"
+  end
+
+  def correctly_configured?
+    true
+  end
+
+  def configuration_hint
+    "hint"
+  end
+
+  def summarize(_content)
+    RESPONSE
+  end
+end