Add HTTP/3 fuzzing harness

[elopez]

This PR implements support for building cURL with QuicTLS, ngtcp2 and nghttp3 and adds a new harness, curl_fuzzer_http3, that fuzzes the HTTP/3 implementation.

scripts/download_ngtcp2.sh includes a patch that adds minimal support on ngtcp2 for using AF_UNIX sockets.

scripts/install_curl.sh includes a patch to disable failing when calling connect() on the QUIC socket (as it will fail if you're using an AF_UNIX socket, like the fuzzer does).