dpctl: add CT Stats for Connections per protocol.

dpctl: add CT Stats for Connections per protocol.

Adds CT stats to report number of connections grouped by
protocol.
By using
 utilities/ovs-appctl dpctl/ct-stats-show
it can display something like:
Connections Stats:
    Total: 1808
        TCP: 1808

With the verbose options:
 utilities/ovs-appctl dpctl/ct-stats-show verbose
it can display:
Connections Stats:
    Total: 2671
        TCP: 2671
          Conn per TCP states:
          [ESTABLISHED]=1000
          [CLOSING]=1
          [TIME_WAIT]=1670

Signed-off-by: Antonio Fischetti <antonio.fischetti@intel.com>
Signed-off-by: Bhanuprakash Bodireddy <bhanuprakash.bodireddy@intel.com>
Co-authored-by: Bhanuprakash Bodireddy <bhanuprakash.bodireddy@intel.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>