HTTP/3: add proxy CONNECT and MASQUE CONNECT-UDP support (ngtcp2 QUIC)

This patch adds two major proxy capabilities to curl (ngtcp2 QUIC):
- HTTP/3 Proxy CONNECT: Tunnel HTTP/1.1 or HTTP/2 traffic through an
  HTTPS proxy that speaks HTTP/3 (QUIC) using the standard CONNECT
  method over an HTTP/3 connection.
- MASQUE CONNECT-UDP: Tunnel HTTP/3 (QUIC) traffic through an HTTP
  proxy (speaking HTTP/1.1, HTTP/2, or HTTP/3) using the extended
  CONNECT method with the CONNECT-UDP protocol (RFC9297 & RFC9298).

Public API additions:
- `CURLPROXY_HTTPS3`: new proxy type constant for HTTP/3 proxy
- `--proxy-http3`: new CLI flag to negotiate HTTP/3 with HTTPS proxy

The implementation adds two new filters:
- `H3-PROXY` - enables negotiating HTTP/3 (QUIC) to the proxy and
  running CONNECT/CONNECT-UDP through that proxy transport.
- `CAPSULE` - dedicated filter inserted between QUIC transport and
  HTTP-PROXY to handle datagram capsule encapsulation/decapsulation.

Here is how the curl filter chaining looks in different scenarios:
- HTTP/3 Proxy CONNECT (tunneling TCP protocols over QUIC proxy):
  conn -> HTTP/1.1 or HTTP/2  -> SSL -> HTTP-PROXY ->
                                 H3-PROXY -> HAPPY-EYEBALLS -> UDP
- MASQUE CONNECT-UDP (tunneling QUIC over any proxy):
  conn -> HTTP/3 -> CAPSULE -> HTTP-PROXY -> H3-PROXY ->
                               HAPPY-EYEBALLS -> UDP
  conn -> HTTP/3 -> CAPSULE -> HTTP-PROXY -> H1-PROXY or H2-PROXY ->
                               SSL -> HAPPY-EYEBALLS -> TCP

- Both features currently require the ngtcp2 QUIC backend.
- Both features are experimental (disabled by default). Enable with
  `--enable-proxy-http3`(autotools) or `-DUSE_PROXY_HTTP3=ON`(CMake).

Tests:
- tests/unit/unit3400.c: Unit tests for capsule protocol encode/decode
- tests/http/test_60_h3_proxy.py: Comprehensive pytest integration suite
- tests/http/testenv/h2o.py: Managing h2o instances with HTTP/1.1, HTTP/2,
  and HTTP/3 (QUIC) listeners, proxy.connect and proxy.connect-udp enabled.

References:
  RFC 9297 - HTTP Datagrams and the Capsule Protocol
  RFC 9298 - Proxying UDP in HTTP
  RFC 9000 §16 — Variable-Length Integer Encoding

Signed-off-by: Aritra Basu <aritrbas+gh@cisco.com>

Closes #21153

lib/vtls/openssl.c

@@ -3713,8 +3713,11 @@ CURLcode Curl_ossl_ctx_init(struct ossl_ctx *octx,
       return result;
   }
 
-  if(data->set.fdebug && data->set.verbose) {
-    /* the SSL trace callback is only used for verbose logging */
+  if(data->set.fdebug && data->set.verbose &&
+     (peer->transport != TRNSPRT_QUIC)) {
+    /* the SSL trace callback is only used for verbose logging;
+     * QUIC connections use a different TLS record format that
+     * ossl_trace cannot handle */
     SSL_CTX_set_msg_callback(octx->ssl_ctx, ossl_trace);
     SSL_CTX_set_msg_callback_arg(octx->ssl_ctx, cf);
   }
@@ -4007,12 +4010,20 @@ static CURLcode ossl_connect_step1(struct Curl_cfilter *cf,
 {
   struct ssl_connect_data *connssl = cf->ctx;
   struct ossl_ctx *octx = (struct ossl_ctx *)connssl->backend;
+  char tls_id[80];
   BIO *bio;
   CURLcode result;
 
   DEBUGASSERT(ssl_connect_1 == connssl->connecting_state);
   DEBUGASSERT(octx);
 
+  if(!connssl->peer.dest) {
+    Curl_ossl_version(tls_id, sizeof(tls_id));
+    result = Curl_ssl_peer_init(&connssl->peer, cf, tls_id, TRNSPRT_TCP);
+    if(result)
+      return result;
+  }
+
   result = Curl_ossl_ctx_init(octx, cf, data, &connssl->peer,
                               connssl->alpn, NULL, NULL,
                               ossl_new_session_cb, cf,

lib/vtls/vtls.c

@@ -1197,6 +1197,7 @@ void Curl_ssl_peer_cleanup(struct ssl_peer *peer)
   Curl_peer_unlink(&peer->dest);
   curlx_safefree(peer->sni);
   curlx_safefree(peer->scache_key);
+  peer->transport = TRNSPRT_NONE;
   peer->type = CURL_SSL_PEER_DNS;
 }
 
@@ -1206,6 +1207,8 @@ static void cf_close(struct Curl_cfilter *cf, struct Curl_easy *data)
   if(connssl) {
     connssl->ssl_impl->close(cf, data);
     connssl->state = ssl_connection_none;
+    connssl->connecting_state = ssl_connect_1;
+    connssl->prefs_checked = FALSE;
     Curl_ssl_peer_cleanup(&connssl->peer);
   }
   cf->connected = FALSE;

lib/vtls/vtls_int.h

@@ -133,9 +133,6 @@ struct ssl_connect_data {
   BIT(input_pending);               /* data for SSL_read() may be available */
 };
 
-#undef CF_CTX_CALL_DATA
-#define CF_CTX_CALL_DATA(cf) ((struct ssl_connect_data *)(cf)->ctx)->call_data
-
 /* Definitions for SSL Implementations */
 
 struct Curl_ssl {
@@ -209,3 +206,9 @@ CURLcode Curl_on_session_reuse(struct Curl_cfilter *cf,
 #endif /* USE_SSL */
 
 #endif /* HEADER_CURL_VTLS_INT_H */
+
+#ifdef USE_SSL
+/* Restore the default SSL filter call_data accessor for unity builds. */
+#undef CF_CTX_CALL_DATA
+#define CF_CTX_CALL_DATA(cf) ((struct ssl_connect_data *)(cf)->ctx)->call_data
+#endif