Summary:
To set `Type Of Service` for IP layer packets ZMQ provides `ZMQ_TOS` socket
option. However this only works for v4 sockets. Considering things are moving to
IPv6 heavily (especially within enterprise networks), ZMQ should support setting
`traffic class` for v6 based on `ZMQ_TOS`.

There is a subtle difference between v4 and v6 in terms of the positioning of
field but TOS has same meaning in both v4 and v6. Linux provides following APIs
for v4/v6 to set TOS field value.
```
// For v4
setsockopt(fd, IPPROTO_IP, IP_TOS, tos, sizeof(tos));

// For v6
setsockopt(fd, IPPROTO_IPV6, IPV6_TCLASS, tos, sizeof(tos));
```

Test Plan:
Make sure Cmake works fine and all tests passes.
Imported this change to `OpenR` project and tested on our local testbed.
Captured some packets exchanged between PUB/SUB and ROUTER/ROUTER sockets
pairs. Verify that `TCLASS` value is set as per expectation.

Tasks: #2208