Though there's already a `createinvoice` command, there are usecases where a
user may want to sign an invoice that they don't yet have the preimage to. For
example, they may have an htlc_accepted plugin that pays to obtain the preimage
from someone else and returns a `{ "result": "resolve", ... }`.
This RPC command addresses this usecase without overly complicating the
semantics of the existing `createinvoice` command.
Changelog-Added: JSON-RPC: `signinvoice` new command to sign BOLT11
invoices.
