feat: Add support for the references in FAQs (#10699)

Currently, it’s unclear whether an FAQ item is generated from a
document, derived from a conversation, or added manually.

This PR resolves the issue by providing visibility into the source of
each FAQ. Users can now see whether an FAQ was generated or manually
added and, if applicable, by whom.

- Move the document_id to a polymorphic relation (documentable).
- Updated the APIs to accommodate the change.
- Update the service to add corresponding references. 
- Updated the specs.

<img width="1007" alt="Screenshot 2025-01-15 at 11 27 56 PM"
src="https://github.com/user-attachments/assets/7d58f798-19c0-4407-b3e2-748a919d14af"
/>

---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>

enterprise/app/views/api/v1/models/captain/_assistant_response.json.jbuilder

@@ -4,13 +4,27 @@ json.assistant do
   json.partial! 'api/v1/models/captain/assistant', formats: [:json], resource: resource.assistant
 end
 json.created_at resource.created_at.to_i
-if resource.document
-  json.document do
-    json.id resource.document.id
-    json.external_link resource.document.external_link
-    json.name resource.document.name
+
+if resource.documentable
+  json.documentable do
+    json.type resource.documentable_type
+
+    case resource.documentable_type
+    when 'Captain::Document'
+      json.id resource.documentable.id
+      json.external_link resource.documentable.external_link
+      json.name resource.documentable.name
+    when 'Conversation'
+      json.id resource.documentable.display_id
+      json.display_id resource.documentable.display_id
+    when 'User'
+      json.id resource.documentable.id
+      json.email resource.documentable.email
+      json.available_name resource.documentable.available_name
+    end
   end
 end
+
 json.id resource.id
 json.question resource.question
 json.updated_at resource.updated_at.to_i