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
"options", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED_NAMED_ARG, "",
                Cat<std::vector<RPCArg>>(
                    {
                        {"add_to_wallet", RPCArg::Type::BOOL, RPCArg::Default{true}, "When false, returns the serialized transaction without broadcasting or adding it to the wallet"},
                        {"fee_rate", RPCArg::Type::AMOUNT, RPCArg::DefaultHint{"not set, fall back to wallet fee estimation"}, "Specify a fee rate in " + CURRENCY_ATOM + "/vB."},
                        {"include_watching", RPCArg::Type::BOOL, RPCArg::DefaultHint{"true for watch-only wallets, otherwise false"}, "Also select inputs which are watch-only.\n"
                                              "Only solvable inputs can be used. Watch-only destinations are solvable if the public key and/or output script was imported,\n"
                                              "e.g. with 'importpubkey' or 'importmulti' with the 'pubkeys' or 'desc' field."},
                        {"inputs", RPCArg::Type::ARR, RPCArg::Default{UniValue::VARR}, "Use exactly the specified inputs to build the transaction. Specifying inputs is incompatible with send_max. A JSON array of JSON objects",
                            {
                                {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
                                {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output number"},
                                {"sequence", RPCArg::Type::NUM, RPCArg::Optional::NO, "The sequence number"},
                            },
                        },
                        {"locktime", RPCArg::Type::NUM, RPCArg::Default{0}, "Raw locktime. Non-0 value also locktime-activates inputs"},
                        {"lock_unspents", RPCArg::Type::BOOL, RPCArg::Default{false}, "Lock selected unspent outputs"},
                        {"psbt", RPCArg::Type::BOOL,  RPCArg::DefaultHint{"automatic"}, "Always return a PSBT, implies add_to_wallet=false."},
                        {"send_max", RPCArg::Type::BOOL, RPCArg::Default{false}, "When true, only use UTXOs that can pay for their own fees to maximize the output amount. When 'false' (default), no UTXO is left behind. send_max is incompatible with providing specific inputs."},
                    },
                    FundTxDoc()
                ),
                "options"
            },
        },
        RPCResult{
            RPCResult::Type::OBJ, "", "",
                {
                    {RPCResult::Type::BOOL, "complete", "If the transaction has a complete set of signatures"},
                    {RPCResult::Type::STR_HEX, "txid", true, "The transaction id for the send. Only 1 transaction is created regardless of the number of addresses."},