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
if (smallest_result_stack.size() != 0) {
        result = std::move(smallest_result_stack);
        return true;
    }
    return false;
}
/**
 * Sign scriptPubKey using signature made with creator.
 * Signatures are returned in scriptSigRet (or returns false if scriptPubKey can't be signed),
 * unless whichTypeRet is TxoutType::SCRIPTHASH, in which case scriptSigRet is the redemption script.
 * Returns false if scriptPubKey could not be completely satisfied.
 */
static bool SignStep(const SigningProvider& provider, const BaseSignatureCreator& creator, const CScript& scriptPubKey,
                     std::vector<valtype>& ret, TxoutType& whichTypeRet, SigVersion sigversion, SignatureData& sigdata)
{
    CScript scriptRet;
    uint160 h160;
    ret.clear();
    std::vector<unsigned char> sig;
    std::vector<valtype> vSolutions;
    whichTypeRet = Solver(scriptPubKey, vSolutions);
    switch (whichTypeRet) {
    case TxoutType::NONSTANDARD:
    case TxoutType::NULL_DATA:
    case TxoutType::WITNESS_UNKNOWN:
        return false;
    case TxoutType::PUBKEY:
        if (!CreateSig(creator, sigdata, provider, sig, CPubKey(vSolutions[0]), scriptPubKey, sigversion)) return false;
        ret.push_back(std::move(sig));
        return true;