Invoices

Research and prototype a library to implement payment invoices

Each hexstody user can have multiple invoices those are either created manually or via API call. An invoice is information required to receive payment from external user to the hexstody account. That includes:

* Currency and Payment method (LN or onchain)
* Address (BTC, ETH) or LN invoice
* Amount to send
* Creation timestamp
* Due timestamp
* External order id (to link the invoice to an order in merchant's system)
* Contact info of payer (email e.x.), optional, for possible refund communication.
* Description
* Optional callback URL from user which we call when the invoice change its state.

Invoices can have several states:
* Created
* Timeout
* Canceled (with reason str field)
* Paid (with proof of payment)

There should be menu in UI to list, view, create (form) and cancel invoices. There should be JSON API for list, view, create and cancel of invoices.