accounts/external, internal/ethapi: fixes for London tx signing (#23274)

Ticket #23273 found a flaw where we were unable to sign legacy-transactions
using the external signer, even if we're still on non-london network. That's
fixed in this PR.

Additionally, I found that even when supplying all parameters, it was impossible
to sign a london-transaction on an unsynched node. It's a pretty common usecase
that someone wants to sign a transaction using an unsynced 'vanilla' node,
providing all necessary data. Our setDefaults, however, insisted on checking the
current block against the config. This PR therefore adds a case, so that if both
MaxPriorityFeePerGas and MaxFeePerGas are provided, we accept them as given.

OBS This PR fixes a regression -- on current master, we are unable to sign a
london-transaction unless the node is synched, which may break scenarios where
geth (or clef) is used as a cold wallet.

Fixes #23273

accounts/external/backend.go

@@ -211,11 +211,14 @@ func (api *ExternalSigner) SignTx(account accounts.Account, tx *types.Transactio
 		To:    to,
 		From:  common.NewMixedcaseAddress(account.Address),
 	}
-	if tx.GasFeeCap() != nil {
+	switch tx.Type() {
+	case types.LegacyTxType, types.AccessListTxType:
+		args.GasPrice = (*hexutil.Big)(tx.GasPrice())
+	case types.DynamicFeeTxType:
 		args.MaxFeePerGas = (*hexutil.Big)(tx.GasFeeCap())
 		args.MaxPriorityFeePerGas = (*hexutil.Big)(tx.GasTipCap())
-	} else {
-		args.GasPrice = (*hexutil.Big)(tx.GasPrice())
+	default:
+		return nil, fmt.Errorf("Unsupported tx type %d", tx.Type())
 	}
 	// We should request the default chain id that we're operating with
 	// (the chain we're executing on)