Doesn't support the hmac-secret extension, silently ignored

[nabijaczleweli]

Noting this down in case anyone else spends a few hours bisecting the issue like I did – this means that, for example:
FIDO_EXT_HMAC_SECRET is accepted but doesn't do anything,
a cert created with fido_cred_set_extensions(cert, FIDO_EXT_HMAC_SECRET); fido_dev_make_cred(dev, cred, nullptr); fido_cred_verify(cred) will fail to verify,
fido_assert_set_extensions(assertion, FIDO_EXT_HMAC_SECRET) will make fido_assert_verify(assertion, 0, COSE_ES256, pubkey_parsed) fail,
fido_dev_get_assert(dev, assertion, nullptr); fido_assert_verify(assertion, 0, COSE_ES256, pubkey_parsed) fails as well.

It wasn't obvious this were the case because the systemd FIDO2 driver uses this extension unconditionally, and both YubiKeys and my Somu support it.