Listen on fraud-sub topic and implement a watcher similar to header-sub-watcher

[zvolin]

• add the fraud-sub topic
• add the fraud_sub_watcher like header_sub_watcher
• optional: create some fraud proof in go node and have some tests where we announce it. We aren't going to be distributing those proofs from our side anytime soon, so we don't need constructing and announcing part in our node itself