Move built-in nodes out to separate projects

[loleg]

The Price Retriever and Mail Sender nodes are built into Proxeus. They should be optional, and their code should be moved to external nodes. Unless their interface is not standard, or it is very difficult to do so.

See:

• https://github.com/ProxeusApp/proxeus-core/blob/main/main/app/price_retriever.go