Proposal: Websocket Payment Tracking

[cryptocode]

This PR attempts to provide an easy and reliable way to get payment notifications, through a Websocket subscription. It's an alternative to block confirmation/pending/balance checks, and supports single-use accounts and block hand-off.

Preliminary documentation: https://github.com/cryptocode/notes/wiki/Websocket-Payment-Tracking

• It adds ledger functionality similar to Add include_only_confirmed to account_balance & account_info #3022 - will move this PR to those ledger functions once they're merged.

[wezrule]

This is really nice!! Having recently had the headache of doing something similar recently I can definitely see the usefulness.

[wezrule]

I also agree it would be nice if wallets supported the "blob" qr code for sending from an account, for instance if the customer needs a refund can get it from the account (assuming the funds are pocketed first), and also for payouts from things like arcade machine.