External inputs are currently only supported for initial transaction creation functions. Extend `solving_data` arguments as necessary to support fee bumping wallet transactions with non-wallet inputs.

https://github.com/bitcoin/bitcoin/pull/17211#issuecomment-897653994
{
    const bool want_psbt = method_name == "psbtbumpfee";
    const std::string incremental_fee{CFeeRate(DEFAULT_INCREMENTAL_RELAY_FEE).ToString(FeeEstimateMode::SAT_VB)};
    return RPCHelpMan{method_name,