Add support for HTTP/3 (IETF QUIC) ("h3" protocol)

[sfionov]

Add support for HTTP/3 filtering.

[TPS]

@ameshkov Maybe #486?

[sfionov]

@TPS No, IETF QUIC and existing QUIC are different protocols.
IETF QUIC is split to QUIC transport (with modified TLS1.3 handshake) and HTTP over QUIC (hq) based on modified HTTP/2.
Google QUIC is more solid and have different Alt-Svc/ALPN protocol name.

[sfionov]

nghq seems to be abandoned. Current implementations:

HTTP/3 QUIC transport:
https://github.com/ngtcp2/ngtcp2

HTTP/3 application protocol:
https://github.com/cloudflare/quiche (Rust, C bindings exist)
https://github.com/ngtcp2/nghttp3