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
RPCResult::Type::OBJ, "", "",
                    {
                        {RPCResult::Type::STR, "psbt", "The base64-encoded partially signed transaction"},
                        {RPCResult::Type::BOOL, "complete", "If the transaction has a complete set of signatures"},
                    }
                },
                RPCExamples{
                    HelpExampleCli("walletprocesspsbt", "\"psbt\"")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    const std::shared_ptr<const CWallet> pwallet = GetWalletForJSONRPCRequest(request);
    if (!pwallet) return UniValue::VNULL;
    const CWallet& wallet{*pwallet};
    
    
    wallet.BlockUntilSyncedToCurrentChain();
    RPCTypeCheck(request.params, {UniValue::VSTR});
    /
    PartiallySignedTransaction psbtx;
    std::string error;
    if (!DecodeBase64PSBT(psbtx, request.params[0].get_str(), error)) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
    }
    /
    int nHashType = ParseSighashString(request.params[2]);
    /
    bool sign = request.params[1].isNull() ? true : request.params[1].get_bool();
    bool bip32derivs = request.params[3].isNull() ? true : request.params[3].get_bool();
    bool finalize = request.params[4].isNull() ? true : request.params[4].get_bool();