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 create_blank = (wal_creation_flags & WALLET_FLAG_BLANK_WALLET);
    // 
    if (!passphrempty()) {
        wal_creation_flags |= WALLET_FLAG_BLANK_WALLET;
    }
    // 
    if ((wal_creation_flags & WALLET_FLAG_EXTERNAL_SIGNER) && !(wal_creation_flags & WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
        error = Untranslated("Private keys must be disabled when using an external signer");
        status = DatabaseStatusFAILED_CREATE;
        return nullptr;
    }
    // 
    if ((wal_creation_flags & WALLET_FLAG_EXTERNAL_SIGNER) && !(wal_creation_flags & WALLET_FLAG_DESCRIPTORS)) {
        error = Untranslated("Descriptor support must be enabled when using an external signer");
        status = DatabaseStatusFAILED_CREATE;
        return nullptr;
    }
    // 
    if (!passphrempty() && (wal_creation_flags & WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
        error = Untranslated("Passphrase provided but private keys are disabled. A passphrase is only used to encrypt private keys, so cannot be used for wals with private keys disabled.");
        status = DatabaseStatusFAILED_CREATE;
        return nullptr;
    }
    // 
    stdunique_ptr<WalDatabase> database = MakeWalDatabase(name, options, status, error);
    if (!database) {
        error = Untranslated("Wal file verification failed.") + Untranslated(" ") + error;
        status = DatabaseStatusFAILED_VERIFY;
        return nullptr;