feat: Add Public APIs for API Channel (#2375)

app/views/public/api/v1/models/_conversation.json.jbuilder

@@ -0,0 +1,10 @@
+json.id resource.display_id
+json.inbox_id resource.inbox_id
+json.contact_last_seen_at resource.contact_last_seen_at.to_i
+json.status resource.status
+json.messages do
+  json.array! resource.messages do |message|
+    json.partial! 'public/api/v1/models/message.json.jbuilder', resource: message
+  end
+end
+json.contact resource.contact