netlink: specs: add conntrack dump and stats dump support

This adds support to dump the connection tracking table
("conntrack -L") and the conntrack statistics, ("conntrack -S").

Example conntrack dump:
tools/net/ynl/pyynl/cli.py --spec Documentation/netlink/specs/conntrack.yaml --dump get
[{'id': 59489769,
  'mark': 0,
  'nfgen-family': 2,
  'protoinfo': {'protoinfo-tcp': {'tcp-flags-original': {'flags': {'maxack',
                                                                   'sack-perm',
                                                                   'window-scale'},
                                                         'mask': set()},
                                  'tcp-flags-reply': {'flags': {'maxack',
                                                                'sack-perm',
                                                                'window-scale'},
                                                      'mask': set()},
                                  'tcp-state': 'established',
                                  'tcp-wscale-original': 7,
                                  'tcp-wscale-reply': 8}},
  'res-id': 0,
  'secctx': {'secctx-name': 'system_u:object_r:unlabeled_t:s0'},
  'status': {'assured',
             'confirmed',
             'dst-nat-done',
             'seen-reply',
             'src-nat-done'},
  'timeout': 431949,
  'tuple-orig': {'tuple-ip': {'ip-v4-dst': '34.107.243.93',
                              'ip-v4-src': '192.168.0.114'},
                 'tuple-proto': {'proto-dst-port': 443,
                                 'proto-num': 6,
                                 'proto-src-port': 37104}},
  'tuple-reply': {'tuple-ip': {'ip-v4-dst': '192.168.0.114',
                               'ip-v4-src': '34.107.243.93'},
                  'tuple-proto': {'proto-dst-port': 37104,
                                  'proto-num': 6,
                                  'proto-src-port': 443}},
  'use': 1,
  'version': 0},
 {'id': 3402229480,

Example stats dump:
tools/net/ynl/pyynl/cli.py --spec Documentation/netlink/specs/conntrack.yaml --dump get-stats
[{'chain-toolong': 0,
  'clash-resolve': 3,
  'drop': 0,
 ....

Changes since last iteration:
 - Address comments from Donald Hunter, in particular, fixup "get" and
   "get-stats" descriptions, the former operation supports both dump
   and normal request (returns a single entry, if found), the latter
   only supports dumps.

Signed-off-by: Florian Westphal <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Florian Westphal