DRAFT: interactive tx construction protocol

In a conversation, ZmnSCPxj shared a link to the challenge-bypass-extension protocol on Github. They noted that the protocol is different from their use-case since it's not client-server based. They explain that in most cases over the internet, the server passively waits for the client to contact it for service. However, in the context of Lightning opening protocols and JoinMarket, the channel opener/taker is the client while the acceptor/maker is the server. ZmnSCPxj says they are still going through the document and will update accordingly.