LUD-21: verify base spec.

author: reneaaron

---

Allow SERVICEs to check whether an invoice has been paid by contacting the LN SERVICE.

A SERVICE can verify a payment was made without access to the user's node. Users can give their LNURL or Lightning Address to a SERVICE and allow the SERVICE to create and verify invoices on their behalf.

This allows users to re-use existing wallets and SERVICEs to facilitate P2P payments without being an intermediary.

LNURL-verify

An optional field verify is added to the callback response:

{
  "status": "OK",
  "routes": [],
  "pr":     "lnbc10...",
  "verify": "https://example.com/verify/894e7f7e...",
}

The field verify provides an URL to a SERVICE to check if the invoice has been settled:

https://example.com/verify/894e7f7e...

Response

{
  "status": "OK",
  "settled": true,
  "preimage": "123456...",
  "pr": "lnbc10..."
}

{
  "status": "OK",
  "settled": false,
  "preimage": null,
  "pr": "lnbc10..."
}

or

{
  "status": "ERROR",
  "reason": "Not found"
}