Nftables: Added xt-compat framework.

- Netfilter NFT compat message parsing.
- Added compat handler for match/target ops.
- Added noop (TCP, UDP) matchers.
- Fixed Return opcode handling;
    The `NFT_RETURN` was rewinding whole-stack instead
    of just the top of stack function.
- Netlink dump request utilities and syscall tests.

PiperOrigin-RevId: 963872533