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 CWalletTx* GetWalletTx(const uint256& hash) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    std::set<uint256> GetTxConflicts(const CWalletTx& wtx) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    /**
     * Return depth of transaction in blockchain:
     * <0  : conflicts with a transaction this deep in the blockchain
     *  0  : in memory pool, waiting to be included in a block
     * >=1 : this many blocks deep in the main chain
     */
    int GetTxDepthInMainChain(const CWalletTx& wtx) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    bool IsTxInMainChain(const CWalletTx& wtx) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet)
    {
        AssertLockHeld(cs_wallet);
        return GetTxDepthInMainChain(wtx) > 0;
    }
    /**
     * @return number of blocks to maturity for this transaction:
     *  0 : is not a coinbase transaction, or is a mature coinbase transaction
     * >0 : is a coinbase transaction which matures in this many blocks
     */
    int GetTxBlocksToMaturity(const CWalletTx& wtx) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    bool IsTxImmatureCoinBase(const CWalletTx& wtx) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
    
    bool CanSupportFeature(enum WalletFeature wf) const override EXCLUSIVE_LOCKS_REQUIRED(cs_wallet) { AssertLockHeld(cs_wallet); return IsFeatureSupported(nWalletVersion, wf); }