Create ALPNIdentifier type to allow bytes or string presentation (#385)

* Create ALPNIdentifier type to allow bytes or string presentation

ALPN IDs aren't guaranteed to be safe text, so allow logging the byte
sequence. However, don't force endpoint to do that if their willing
to do some safety checking or safe encoding.

Closes #368

---------

Co-authored-by: Robin Marx <rmarx@akamai.com>

draft-ietf-quic-qlog-quic-events.md

@@ -529,22 +529,27 @@ Intended use:
 
 ## alpn_information {#quic-alpninformation}
 
-The `alpn_information` event support application level protocol negotiation over
-the QUIC transport; see {{Section 7.4 of QUIC-TRANSPORT}}. It has Core
-importance level; see {{Section 9.2 of QLOG-MAIN}}.
+The `alpn_information` event supports Application-Layer Protocol Negotiation
+(ALPN) over the QUIC transport; see {{?RFC7301}} and {{Section 7.4 of
+QUIC-TRANSPORT}}. It has Core importance level; see {{Section 9.2 of
+QLOG-MAIN}}.
+
+QUIC endpoints are configured with a list of supported ALPN identifiers. Clients send the list in a TLS ClientHello, and servers match against their list. On success, a single ALPN identifier is chosen and sent back in a TLS ServerHello. If no match is found, the connection is closed.
 
-QUIC implementations each have their own list of application level protocols and
-versions thereof they support. The client includes a list of their supported
-options in its first initial as part of the TLS Application Layer Protocol
-Negotiation (alpn) extension. If there are common option(s), the server chooses
-the most optimal one and communicates this back to the client. If not, the
-connection is closed.
+ALPN identifiers are byte sequences, that may be possible to present as UTF-8.
+The `ALPNIdentifier`` type supports either format. Implementations SHOULD log at
+least one format, but MAY log both or none.
 
 ~~~ cddl
 QUICALPNInformation = {
-    ? server_alpns: [* text]
-    ? client_alpns: [* text]
-    ? chosen_alpn: text
+    ? server_alpns: [* ALPNIdentifier]
+    ? client_alpns: [* ALPNIdentifier]
+    ? chosen_alpn: ALPNIdentifier
+}
+
+ALPNIdentifier = {
+  ? byte_value: hexstring
+  ? string_value: text
 }
 ~~~
 {: #quic-alpninformation-def title="QUICALPNInformation definition"}