Create an invoice

Mints a FRESH invoice every time. There is no get-or-create and there cannot be one: `/addresses` is idempotent on a reference you supply, but an invoice is amount-fixed and single-use, so the same trick does not apply. **A request that times out and is retried mints a SECOND invoice.** That is cheap rather than dangerous - an unpaid invoice expires on its own, and only one can ever be paid, because the credit is keyed on the payment hash. The cost is showing the wrong one to a payer, so prefer one mint per payment attempt over blind retries.

curl -X POST "https://example.com/lightning/invoices" \  -H "Content-Type: application/json" \  -d '{    "amount_sat": 25000  }'

{  "data": {    "invoice": "string",    "amount_sat": 0,    "expires_at": "2019-08-24T14:15:22Z"  }}