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. 
struct TxSize {
    int64_t vsize{-1};
    int64_t weight{-1};
};
/** Calculate the size of the transaction using CoinControl to determine
 * whether to expect signature grinding when calculating the size of the input spend. */
TxSize CalculateMaximumSignedTxSize(const CTransaction& tx, const CWallet* wallet, const std::vector<CTxOut>& txouts, const CCoinControl* coin_control = nullptr);
TxSize CalculateMaximumSignedTxSize(const CTransaction& tx, const CWallet* wallet, const CCoinControl* coin_control = nullptr) EXCLUSIVE_LOCKS_REQUIRED(wallet->cs_wallet);
/**
 * COutputs available for spending, stored by OutputType.
 * This struct is really just a wrapper around OutputType vectors with a convenient
 * method for concatenating and returning all COutputs as one vector.
 *
 * Size(), Clear(), Erase(), Shuffle(), and Add() methods are implemented to
 * allow easy interaction with the struct.
 */
struct CoinsResult {
    std::map<OutputType, std::vector<COutput>> coins;
    
    std::vector<COutput> All() const;
    /** The following methods are provided so that CoinsResult can mimic a vector,
     * i.e., methods can work with individual OutputType vectors or on the entire object */
    size_t Size() const;
    
    size_t TypesCount() const { return coins.size(); }
    void Clear();
    void Erase(const std::unordered_set<COutPoint, SaltedOutpointHasher>& coins_to_remove);
    void Shuffle(FastRandomContext& rng_fast);
    void Add(OutputType type, const COutput& out);
    CAmount GetTotalAmount() { return total_amount; }