vquic: ngtcp2 + openssl support

With the new addition of OpenSSL QUIC API support and the support in
ngtcp2 main branch, make the necessary adjustments in curl to support
this combination.

- add support in configure.ac to detect the feature OPENSSL_QUIC_API2 in
  openssl
- initialise ngtcp2 properly in this combination
- add a Curl_vquic_init() for global initialisation that ngtcp2 likes
  for performance reasons
- add documentation on how to build in docs/HTTP3.md
- add CI testing in http3-linux.yml

Assisted-by: Viktor Szakats
Closes #17027

lib/easy.c

@@ -49,6 +49,7 @@
 #include "transfer.h"
 #include "vtls/vtls.h"
 #include "vtls/vtls_scache.h"
+#include "vquic/vquic.h"
 #include "url.h"
 #include "getinfo.h"
 #include "hostip.h"
@@ -170,6 +171,11 @@ static CURLcode global_init(long flags, bool memoryfuncs)
     goto fail;
   }
 
+  if(!Curl_vquic_init()) {
+    DEBUGF(fprintf(stderr, "Error: Curl_vquic_init failed\n"));
+    goto fail;
+  }
+
   if(Curl_win32_init(flags)) {
     DEBUGF(fprintf(stderr, "Error: win32_init failed\n"));
     goto fail;