Payment URI with fallback

[frechfish]

Some payment requests contain an on-chain fallback address. This makes these payment requests compatible with (old) main-chain wallets and Lightning wallets. The user can then choose how they want to pay or switch to on-chain if Lightning fails.

The address format looks like this:
bitcoin:bc1xxxxx?lightning=lnurl1xxxxx

I think it makes sense to introduce this for Liquid addresses as well, so that users can choose whether they want to pay via the main-chain or Liquid.

The address format could look like this:
bitcoin:bc1xxxxx?lightning=lnurl1xxxxx&liquid=Glxxxxx

It would make sense to ask users when creating invoices whether they want to embed main chain, liquid, or both in the payment request.

Somewhat related: #81

[ardier16]

@frechfish thanks for the suggestion, and apologies for the delayed response! The app currently creates separate network-specific payment requests and does not generate a combined URI like the proposed format. We’ll keep this open as a feature request.