Finishing my chirping wallet PR.

This PR basically introduces a wallet template so to add a new wallet to SN, we only need to fill out this template. No complicated, error-prone changes across multiple files required anymore.

I might even include LNbits for receiving to test how well it actually works in practice. It worked well for the existing wallets. But all existing wallets only support receiving or sending, but not both yet. So LNbits would be the first wallet that supports sending and receiving at the same time then.

Let's see.