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
{
    return RPCHelpMan{"fundrawtransaction",
                "\nIf the transaction has no inputs, they will be automatically selected to meet its out value.\n"
                "It will add at most one change output to the outputs.\n"
                "No existing outputs will be modified unless \"subtractFeeFromOutputs\" is specified.\n"
                "Note that inputs which were signed may need to be resigned after completion since in/outputs have been added.\n"
                "The inputs added will not be signed, use signrawtransactionwithkey\n"
                "or signrawtransactionwithwallet for that.\n"
                "All existing inputs must either have their previous output transaction be in the wallet\n"
                "or be in the UTXO set. Solving data must be provided for non-wallet inputs.\n"
                "Note that all inputs selected must be of standard form and P2SH scripts must be\n"
                "in the wallet using importaddress or addmultisigaddress (to calculate fees).\n"
                "You can see whether this is the case by checking the \"solvable\" field in the listunspent output.\n"
                "Only pay-to-pubkey, multisig, and P2SH versions thereof are currently supported for watch-only\n",
                {
                    {"hexstring", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The hex string of the raw transaction"},
                    {"options", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED_NAMED_ARG, "for backward compatibility: passing in a true instead of an object will result in {\"includeWatching\":true}",