tx-psbt: pass in the witness script (if known) when adding an input

Update the `bitcoin_tx_add_input` interface to accept a witness script
and or scriptPubkey.

We save the amount + witness script + witness program (if known) to
the PSBT object for a transaction when creating an input.

channeld/commit_tx.h

@@ -48,6 +48,7 @@ struct bitcoin_tx *commit_tx(const tal_t *ctx,
 			     const struct bitcoin_txid *funding_txid,
 			     unsigned int funding_txout,
 			     struct amount_sat funding,
+			     const u8 *funding_wscript,
 			     enum side opener,
 			     u16 to_self_delay,
 			     const struct keyset *keyset,