feature request: http/3 client

[liudongmiao]

There is udp request on libevent. Now, it's the time for udp and quic.

For client, there would be two parts:

• udp client
• quic over udp client, requires ssl library, and maybe nghttp3

For server, there would be two parts:

• udp server
• quic over udp server, requires ssl library -- do we need to support this?

In the low-level view, udp is nothing different from tcp in the event of libevent.
However, if we want to offer extra bufferevent like bufferevent_socket_new / bufferevent_socket_connect for tcp, we should offer.

[liudongmiao]

LiteSpeed QUIC is a quic library, and use libevent in examples.