eap-tls: Add support for TLS 1.3

As defined in RFC 9190, a "protected success indication" (0x00) is sent
from the server to the client over the TLS connection when using TLS 1.3.

The client responds with an empty EAP message, which is interpreted as
acknowledgement in our stack.

If we ever support session resumption with tunneled methods such as
EAP-TTLS, we'd have to send such an indication there too.

src/libtls/tls_eap.c

@@ -369,6 +369,9 @@ METHOD(tls_eap_t, process, status_t,
 	}
 	else
 	{
+		/* note that with TLS 1.3 the client sends an empty EAP packet after the
+		 * server sent the "protected success indication" over the TLS
+		 * connection, which is interpreted here as an ACK packet */
 		if (in.len == sizeof(eap_tls_packet_t))
 		{
 			DBG2(DBG_TLS, "received %N acknowledgment packet",