socket option: support `IPV6_TCLASS`

[matttbe]

Pre-requisites

• A similar idea has not been reported before.
• mptcp.dev website does not cover my case.
• The wiki doesn't cover my case.

Description

We are supporting IP_TOS (IPv4 specific) for a while, but not IPV6_TCLASS which is the same idea for IPv6.

Solution

Support it, with a validation test (packetdrill and eventually in the selftests as well: similar to what we have with IP_TOS)

Considered alternatives

We don't want to force people to use IPv4 only :)

Additional context

No response