Add support for extensions and attestedCredentialData in assertion response

[sirkrypt0]

We currently do not support the extensions and attestedCredentialData in the authenticator data structure, as specified in WebAuthn § 6.1 Authenticator Data and CTAP 2.1 § 6.2.2 authenticatorGetAssertion Algorithm.

This is mostly due to its variable length, which makes it hard to store in fixed size stack-allocated structures, and also that we didn't need it for our PoC.

However, it might be desirable in the future to receive this data (such as extension outputs) and parse it in the relying party.

Implementation Note

The struct is defined here:

libmicrofido2/include/assertion.h

Lines 69 to 74 in bb3678d

	typedef struct fido_assert_auth_data {
	uint8_t rp_id_hash[ASSERTION_AUTH_DATA_RPID_HASH_LEN];
	fido_assert_auth_data_flags_t flags;
	uint32_t sign_count;
	// TODO: extensions and attestedCredentialData not supported for now.
	} fido_assert_auth_data_t;

The processing may happen here:

libmicrofido2/src/assertion.c

Lines 479 to 489 in bb3678d

	if (fido_check_flags(reply->auth_data.flags, assert->opt) < 0) {
	fido_log_debug("%s: fido_check_flags", __func__);
	return FIDO_ERR_INVALID_PARAM;
	}
	// TODO: Extensions not supported for now.
	if (fido_check_rp_id(&(assert->rp_id), reply->auth_data.rp_id_hash) != 0) {
	fido_log_debug("%s: fido_check_rp_id", __func__);
	return FIDO_ERR_INVALID_PARAM;
	}