ovn-controller: Add 'dns_lookup' action

ovn-controller: Add 'dns_lookup' action

This patch adds a new OVN action 'dns_lookup' to support native DNS.
ovn-controller parses this action and adds a NXT_PACKET_IN2
OF flow with 'pause' flag set.

A new table 'DNS' is added in the SB DB to look up and resolve
the DNS queries. When a valid DNS packet is received by
ovn-controller, it looks up the DNS name in the 'DNS' table
and if successful, it frames a DNS reply, resumes the packet
and stores 1 in the 1-bit subfield. If the packet is invalid
or cannot be resolved, it resumes the packet without any
modifications and stores 0 in the 1-bit subfield.

reg0[4] = dns_lookup(); next;

An upcoming patch will use this action and adds logical flows.

Signed-off-by: Numan Siddique <nusiddiq@redhat.com>
Acked-by: Gurucharan Shetty <guru@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>