feat: Add support for persistent copilot threads and messages (#11489)

The agents can see the previous conversations with the copilot if needed
with this change. We would have to cleanup the data after a while. For
now, that is not considered.

This PR adds:
- A new model for copilot_threads (intentionally named thread instead of
conversation to avoid confusion), copilot_messages
- Add the controller to fetch previous threads and messages.

enterprise/app/views/api/v1/accounts/captain/copilot_messages/index.json.jbuilder

@@ -0,0 +1,8 @@
+json.payload do
+  json.array! @copilot_messages do |message|
+    json.id message.id
+    json.message message.message
+    json.message_type message.message_type
+    json.created_at message.created_at.to_i
+  end
+end

enterprise/app/views/api/v1/accounts/captain/copilot_threads/index.json.jbuilder

@@ -0,0 +1,12 @@
+json.payload do
+  json.array! @copilot_threads do |thread|
+    json.id thread.id
+    json.title thread.title
+    json.uuid thread.uuid
+    json.created_at thread.created_at.to_i
+    json.user do
+      json.id thread.user.id
+      json.name thread.user.name
+    end
+  end
+end