FR: support legacy retail qr codes

[wrapnuts]

Describe your enhancement idea

• moneybadger is an LSP in ZA and wallet that helps translate legacy retail qr codes for lightning payments
• some lightning wallets, including blink already merged support for legacy retail qr codes
• it would be awesome if cashu.me would support this too

merge request in blink for reference:

GaloyMoney/galoy-client#331

[DSanich]

@wrapnuts hello! I found this issue interesting and I proposed my own solution, inspired by the example you provided.

#487