Initial support for RFC 9298 "Proxying UDP in HTTP" (#203)

* Add support for RFC 9298 in :protocol extension

* bugfix: Properly encode :protocol header

Previously, the `:protocol` header wouldn't be set when the `Protocol`
extension was active for a `Request`.

This was most likely because the Webtransport PR (the PR that introduced
the `Protocol` extension) did not include a client implementation and
thus did not test this.

---------

h3/src/ext.rs

@@ -20,11 +20,23 @@ pub struct Protocol(ProtocolInner);
 impl Protocol {
     /// WebTransport protocol
     pub const WEB_TRANSPORT: Protocol = Protocol(ProtocolInner::WebTransport);
+    /// RFC 9298 protocol
+    pub const CONNECT_UDP: Protocol = Protocol(ProtocolInner::ConnectUdp);
+
+    /// Return a &str representation of the `:protocol` pseudo-header value
+    #[inline]
+    pub fn as_str(&self) -> &str {
+        match self.0 {
+            ProtocolInner::WebTransport => "webtransport",
+            ProtocolInner::ConnectUdp => "connect-udp",
+        }
+    }
 }
 
 #[derive(Copy, PartialEq, Debug, Clone)]
 enum ProtocolInner {
     WebTransport,
+    ConnectUdp,
 }
 
 /// Error when parsing the protocol
@@ -36,6 +48,7 @@ impl FromStr for Protocol {
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s {
             "webtransport" => Ok(Self(ProtocolInner::WebTransport)),
+            "connect-udp" => Ok(Self(ProtocolInner::ConnectUdp)),
             _ => Err(InvalidProtocol),
         }
     }

h3/src/proto/headers.rs

@@ -181,6 +181,10 @@ impl Iterator for HeaderIter {
             if let Some(status) = pseudo.status.take() {
                 return Some((":status", status.as_str()).into());
             }
+
+            if let Some(protocol) = pseudo.protocol.take() {
+                return Some((":protocol", protocol.as_str().as_bytes()).into());
+            }
         }
 
         self.pseudo = None;