Nftables: Support xt_conntrack compatibility match.

Added:
- `xt_conntrack` extension support to match packets based
   on conntrack state(new, established, reply, invalid).
- XTConntrackMtinfo1, XTConntrackMtinfo2, and XTConntrackMtinfo3 ABI support.
- Conntrack match evaluation unit tests and syscall test.

Ref: net/netfilter/xt_conntrack.c
PiperOrigin-RevId: 963976341