Enrich invoice QR with reference to identify payments using system.remark

[brenzi]

Shops send out invoices, but currently, they can only guess who the sender is by matching time and amount.

We could add a field to the QR code, i.e. invoice reference number, and upon payment, the add adds a system.remark applying that field using a utility.batch together with the balance transfer call

See also: https://forum.polkadot.network/t/referendum-234-data-availability-retroactive-funding-and-on-chain-invoices/3421