Since #13666 our fee calculation assumes that ECSDA signatures are a maximum of 71 bytes, rather than 72. However, not all hardware wallets implement R-value grinding.

This can cause us to underpay fees when using an external signer. That's especially problematic when the user picks 1 sat/vbyte as the fee, because it won't even get into our own mempool.

The easiest solution would be to modify `ExternalSignerScriptPubKeyMan` to assume 72 bytes for ECSDA signatures.

Slightly more advanced would be to expand HWI to keep track of which devices and firmware versions (if any) support R-value grinding. We'd then have to store that in the wallet in some new field. Personally I'd rather work on better Taproot support, which thanks to Schnorr avoids this issue altogether. 
const CoinFilterParams& params = {}) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet);
/**
 * Wrapper function for AvailableCoins which skips the `feerate` and `CoinFilterParams::only_spendable` parameters. Use this function
 * to list all available coins (e.g. listunspent RPC) while not intending to fund a transaction.
 */
CoinsResult AvailableCoinsListUnspent(const CWallet& wallet, const CCoinControl* coinControl = nullptr, CoinFilterParams params = {}) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet);
CAmount GetAvailableBalance(const CWallet& wallet, const CCoinControl* coinControl = nullptr);
/**
 * Find non-change parent output.
 */
const CTxOut& FindNonChangeParentOutput(const CWallet& wallet, const COutPoint& outpoint) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet);
/**
 * Return list of available coins and locked coins grouped by non-change output address.
 */
std::map<CTxDestination, std::vector<COutput>> ListCoins(const CWallet& wallet) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet);
std::vector<OutputGroup> GroupOutputs(const CWallet& wallet, const std::vector<COutput>& outputs, const CoinSelectionParams& coin_sel_params, const CoinEligibilityFilter& filter, bool positive_only);
/**
 * Attempt to find a valid input set that preserves privacy by not mixing OutputTypes.
 * `ChooseSelectionResult()` will be called on each OutputType individually and the best
 * the solution (according to the waste metric) will be chosen. If a valid input cannot be found from any
 * single OutputType, fallback to running `ChooseSelectionResult()` over all available coins.
 *
 * param@[in]  wallet                    The wallet which provides solving data for the coins
 * param@[in]  nTargetValue              The target value
 * param@[in]  eligilibity_filter        A filter containing rules for which coins are allowed to be included in this selection
 * param@[in]  available_coins           The struct of coins, organized by OutputType, available for selection prior to filtering
 * param@[in]  coin_selection_params     Parameters for the coin selection
 * param@[in]  allow_mixed_output_types  Relax restriction that SelectionResults must be of the same OutputType
 * returns                               If successful, a SelectionResult containing the input set
 *                                       If failed, returns (1) an empty error message if the target was not reached (general "Insufficient funds")