lightningd: Add signinvoice to sign a BOLT11 invoice.

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.

tests/test_invoices.py

@@ -533,6 +533,19 @@ def test_waitanyinvoice(node_factory, executor):
         l2.rpc.waitanyinvoice('non-number')
 
 
+def test_signinvoice(node_factory, executor):
+    # Setup
+    l1, l2 = node_factory.line_graph(2)
+
+    # Create an invoice for l1
+    inv1 = l1.rpc.invoice(1000, 'inv1', 'inv1')['bolt11']
+    assert l1.rpc.decodepay(inv1)['payee'] == l1.info['id']
+
+    # Have l2 re-sign the invoice
+    inv2 = l2.rpc.signinvoice(inv1)['bolt11']
+    assert l1.rpc.decodepay(inv2)['payee'] == l2.info['id']
+
+
 def test_waitanyinvoice_reversed(node_factory, executor):
     """Test waiting for invoices, where they are paid in reverse order
     to when they are created.