Describe the feature

DiffServ defines classes of traffic which can be configured for a socket via a DSCP value.
Zenoh could support the configuration of a DSCP class in the same way than other socket options can be configured on endpoints.
e.g.: "tcp/192.168.0.1:7447#dscp=48".

Note that the way to set DSCP class differ between IPv4 and IPv6:

• IPv4: it can be set via ToS, calling setsockopt(sockfd, IPPROTO_IP, IP_TOS,...).
  In Rust, call socket2::Socket::set_tos(&self, tos: u32)
• IPv6: it can be set via Traffic Class, calling setsockopt(sockfd, IPPROTO_IPV6, IPV6_TCLASS,...).
  In Rust, it seems to be implemented only for UNIX in socket2::sys::unix::Socket::set_tclass_v6(&self, tclass: u32)