Allow sending lightning sats to nodes that accept keysend payments

[blankscreengithub]

I think it's a game changer for operability and greater adoption.

1. Coffee shop has public key as QR code at checkout.
2. I open my Phoenix Wallet
3. I enter the amount to send
4. I scan the QR code
5. Barista at terminal sees my transaction come in
6. I get my goods

No invoice needed. This would also work for small shops, craft vendors, garage sales, etc.

[t-bast]

Addressing this scenario is important, but it's unclear yet how it should be addressed.
Keysend was never standardized and is being deprecated (in favor of AMP but there is no spec proposal describing it yet unless I missed it @cfromknecht ?).
Offers is another attempt at addressing this issue but it has only been implemented by c-lightning so far.

[Kixunil]

@blankscreengithub while I agree that sending to node ID is useful, I believe your scenario is not motivating. Having such scheme would have two significant problems:

1. If two people are attempting to pay at the same time and one of them fails, there's no way to tell which one paid.
2. On-chain transfers in this way had many problems of people sending incorrect amount. This was a huge pain in the ass for merchants. Invoices fix this but your scenario destroys this solution.

Keysend is useful for donations and sending between trusted friends.

[fresheneesz]

It sounds like Offers solves this use case pretty elegantly. Is there a plan to implement it in Phoenix yet?