Check SNI extension is used on H3

[maskit]

QUIC version 1 uses TLS version 1.3 or greater as its handshake
protocol. HTTP/3 clients MUST support a mechanism to indicate the
target host to the server during the TLS handshake. If the server is
identified by a domain name ([DNS-TERMS]), clients MUST send the
Server Name Indication (SNI; [RFC6066]) TLS extension unless an
alternative mechanism to indicate the target host is used.

https://www.rfc-editor.org/rfc/rfc9114.html#section-3.2-2

What to do on the server side doesn't seem to be defined, but we can probably use H3_GENERAL_PROTOCOL_ERROR and close the connection if a client does not use SNI extension.

Note that this is from H3 spec and QUIC does not require use of SNI extension (i.e. we should do this check in proxy/http3).