vquic: ngtcp2 + openssl support

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