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. 
bool m_spend_zero_conf_change{DEFAULT_SPEND_ZEROCONF_CHANGE};
    bool m_signal_rbf{DEFAULT_WALLET_RBF};
    bool m_allow_fallback_fee{true}; 
    CFeeRate m_min_fee{DEFAULT_TRANSACTION_MINFEE}; 
    /**
     * If fee estimation does not have enough data to provide estimates, use this fee instead.
     * Has no effect if not using fee estimation
     * Override with -fallbackfee
     */
    CFeeRate m_fallback_fee{DEFAULT_FALLBACK_FEE};
     /** If the cost to spend a change output at this feerate is greater than the value of the
      * output itself, just drop it to fees. */
    CFeeRate m_discard_rate{DEFAULT_DISCARD_FEE};
    /** When the actual feerate is less than the consolidate feerate, we will tend to make transactions which
     * consolidate inputs. When the actual feerate is greater than the consolidate feerate, we will tend to make
     * transactions which have the lowest fees.
     */
    CFeeRate m_consolidate_feerate{DEFAULT_CONSOLIDATE_FEERATE};
    
    CAmount m_max_aps_fee{DEFAULT_MAX_AVOIDPARTIALSPEND_FEE}; 
    OutputType m_default_address_type{DEFAULT_ADDRESS_TYPE};
    /**
     * Default output type for change outputs. When unset, automatically choose type
     * based on address type setting and the types other of non-change outputs
     * (see -changetype option documentation and implementation in
     * CWallet::TransactionChangeType for details).
     */
    std::optional<OutputType> m_default_change_type{};
    
    CAmount m_default_max_tx_fee{DEFAULT_TRANSACTION_MAXFEE};