chanfunding: support p2tr input fee calculation

With this commit we support fee calculation in coin selection for p2tr
inputs. We assume that coins in our UTXO selection are only BIP0086
coins. Any other input types with different spend paths won't be
selected by the wallet assembler.

lnwallet/chanfunding/coin_select.go

@@ -79,6 +79,11 @@ func calculateFees(utxos []Coin, feeRate chainfee.SatPerKWeight) (btcutil.Amount
 		case txscript.IsPayToScriptHash(utxo.PkScript):
 			weightEstimate.AddNestedP2WKHInput()
 
+		case txscript.IsPayToTaproot(utxo.PkScript):
+			weightEstimate.AddTaprootKeySpendInput(
+				txscript.SigHashDefault,
+			)
+
 		default:
 			return 0, 0, &errUnsupportedInput{utxo.PkScript}
 		}