signpsbt: add signonly parameter to restrict/enforce what inputs to sign.

This is an extra safety check for dual funding, where we only want to sign
the inputs we provided!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: JSON-RPC: `signpsbt` takes an optional `signonly` array to limit what inputs to sign.

contrib/pyln-client/pyln/client/lightning.py

@@ -1155,12 +1155,13 @@ class LightningRpc(UnixDomainSocketRpc):
         }
         return self.call("utxopsbt", payload)
 
-    def signpsbt(self, psbt):
+    def signpsbt(self, psbt, signonly=None):
         """
         Add internal wallet's signatures to PSBT
         """
         payload = {
             "psbt": psbt,
+            "signonly": signonly,
         }
         return self.call("signpsbt", payload)