echo: add echo server

[Spiffyk]

This commit adds an echo server that reflects queries back at the sender. May be used for determining the baseline performance of QUIC (or, at least, of the quic-go implementation in combination with the client's QUIC implementation).

[Spiffyk]

I have updated quic-go, which now supports 0-RTT (supported by the in-development version of DNS Shotgun with QUIC support - I use this echo server to test that). It now however requires Go >=1.20 - is that okay?

Another improvement is the addition of the -keylog string flag that makes the server (both echo and proxy) write TLS keys into the specified file, which is useful for Wireshark analysis.