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/capsule.h

@@ -0,0 +1,77 @@
+#ifndef HEADER_CURL_CAPSULE_H
+#define HEADER_CURL_CAPSULE_H
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ * SPDX-License-Identifier: curl
+ *
+ ***************************************************************************/
+
+#include "curl_setup.h"
+
+#if !defined(CURL_DISABLE_PROXY) && !defined(CURL_DISABLE_HTTP)
+
+#include "curlx/dynbuf.h"
+#include "bufq.h"
+
+/* HTTP Capsule constants */
+#define HTTP_CAPSULE_HEADER_MAX_SIZE    10
+
+/* HTTP Capsule function prototypes */
+
+/**
+ * Write the capsule header (type + varint length + context ID) into `hdr`.
+ * @param hdr         Output buffer (must be >= HTTP_CAPSULE_HEADER_MAX_SIZE)
+ * @param hdrlen      Size of `hdr` in bytes
+ * @param payload_len Length of the UDP payload that follows
+ * @return Number of header bytes written, or 0 on error
+ */
+size_t Curl_capsule_encap_udp_hdr(uint8_t *hdr, size_t hdrlen,
+                                  size_t payload_len);
+
+/**
+ * Encapsulate UDP payload into HTTP Datagram capsule format
+ * @param dyn   Dynamic buffer to write capsule to
+ * @param buf   Payload buffer
+ * @param blen  Payload buffer length
+ * @return CURLE_OK on success, error code on failure
+ */
+CURLcode Curl_capsule_encap_udp_datagram(struct dynbuf *dyn,
+                                         const void *buf, size_t blen);
+
+/**
+ * Process one UDP capsule from buffer into raw datagram payload bytes.
+ * @param cf        Connection filter
+ * @param data      Easy handle
+ * @param recvbufq  Buffer queue containing capsule data
+ * @param buf       Output buffer for one datagram payload
+ * @param len       Size of output buffer in bytes
+ * @param err       Error code output
+ * @return Number of payload bytes written. Check `err` for status.
+ */
+size_t Curl_capsule_process_udp_raw(struct Curl_cfilter *cf,
+                                    struct Curl_easy *data,
+                                    struct bufq *recvbufq,
+                                    unsigned char *buf, size_t len,
+                                    CURLcode *err);
+
+#endif /* !CURL_DISABLE_PROXY && !CURL_DISABLE_HTTP */
+
+#endif /* HEADER_CURL_CAPSULE_H */