feat: preimages for incoming payments, fundingsource saves preimage on create_invoice (#3085)

Co-authored-by: Vlad Stan <stan.v.vlad@gmail.com>

tests/regtest/test_real_invoice.py

@@ -101,6 +101,8 @@ async def test_create_real_invoice(client, adminkey_headers_from, inkey_headers_
         balance = await get_node_balance_sats()
         assert balance - prev_balance == create_invoice.amount
 
+        assert payment_status.get("preimage") is not None
+
         # exit out of infinite loop
         raise FakeError()