HTTP3: initial (experimental) support

USe configure --with-ngtcp2 or --with-quiche

Using either option will enable a HTTP3 build.
Co-authored-by: Alessandro Ghedini <alessandro@ghedini.me>

Closes #3500

src/tool_cfgable.h

@@ -258,6 +258,7 @@ struct OperationConfig {
                                      0 is valid. default: CURL_HET_DEFAULT. */
   bool haproxy_protocol;          /* whether to send HAProxy protocol v1 */
   bool disallow_username_in_url;  /* disallow usernames in URLs */
+  bool h3direct;                  /* go HTTP/3 directly */
   struct GlobalConfig *global;
   struct OperationConfig *prev;
   struct OperationConfig *next;   /* Always last in the struct */

src/tool_getparam.c

@@ -200,6 +200,7 @@ static const struct LongShort aliases[]= {
   {"01",  "http1.1",                 ARG_NONE},
   {"02",  "http2",                   ARG_NONE},
   {"03",  "http2-prior-knowledge",   ARG_NONE},
+  {"04",  "http3-direct",            ARG_NONE},
   {"09",  "http0.9",                 ARG_BOOL},
   {"1",  "tlsv1",                    ARG_NONE},
   {"10",  "tlsv1.0",                 ARG_NONE},
@@ -1189,10 +1190,14 @@ ParameterError getparameter(const char *flag, /* f or -long-flag */
         /* HTTP version 2.0 */
         config->httpversion = CURL_HTTP_VERSION_2_0;
         break;
-      case '3':
+      case '3': /* --http2-prior-knowledge */
         /* HTTP version 2.0 over clean TCP*/
         config->httpversion = CURL_HTTP_VERSION_2_PRIOR_KNOWLEDGE;
         break;
+      case '4': /* --http3-direct */
+        /* HTTP version 3 over QUIC - at once */
+        config->h3direct = toggle;
+        break;
       case '9':
         /* Allow HTTP/0.9 responses! */
         config->http09_allowed = toggle;

src/tool_help.c

@@ -191,6 +191,8 @@ static const struct helptxt helptext[] = {
    "Use HTTP 2"},
   {"    --http2-prior-knowledge",
    "Use HTTP 2 without HTTP/1.1 Upgrade"},
+  {"    --http3-direct",
+   "Use HTTP v3"},
   {"    --ignore-content-length",
    "Ignore the size of the remote resource"},
   {"-i, --include",
@@ -530,6 +532,7 @@ static const struct feat feats[] = {
   {"CharConv",       CURL_VERSION_CONV},
   {"TLS-SRP",        CURL_VERSION_TLSAUTH_SRP},
   {"HTTP2",          CURL_VERSION_HTTP2},
+  {"HTTP3",          CURL_VERSION_HTTP3},
   {"UnixSockets",    CURL_VERSION_UNIX_SOCKETS},
   {"HTTPS-proxy",    CURL_VERSION_HTTPS_PROXY},
   {"MultiSSL",       CURL_VERSION_MULTI_SSL},

src/tool_operate.c

@@ -1118,6 +1118,9 @@ static CURLcode create_transfers(struct GlobalConfig *global,
         if(config->tcp_fastopen)
           my_setopt(curl, CURLOPT_TCP_FASTOPEN, 1L);
 
+        if(config->h3direct)
+          my_setopt(curl, CURLOPT_H3, CURLH3_DIRECT);
+
         /* where to store */
         my_setopt(curl, CURLOPT_WRITEDATA, per);
         my_setopt(curl, CURLOPT_INTERLEAVEDATA, per);