netfilter: nft_ct: Add ct id support

The 'id' key returns the unique id of the conntrack entry as returned
by nf_ct_get_id().

Signed-off-by: Brett Mastbergen <bmastbergen@untangle.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>