feat: Add a pre-chat form on widget (#1769)

app/views/api/v1/widget/conversations/create.json.jbuilder

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