statsclient: added symlinks

statsclient: added symlinks

Symlink support based on https://gerrit.fd.io/r/c/vpp/+/31636

Added new stat types `CounterStat` and `CombinedCounterStat` which
represent simple/combined value of a single item (interface, node)
as a array of values by workers.

Example:
/if/names                      NameVector [local0 tap0 tap1]
/if/ip6               SimpleCounterVector [[0 9 0] [0 25 3] [0 0 60] [0 0 0]]
/interfaces/tap0/ip6  SimpleCounterVector [[9] [25] [0] [0]]
/interfaces/tap1/ip6  SimpleCounterVector [[0] [3] [60] [0]]

Field `Symlink` added to StatEntry to mark symlink stats.

For stats v2 only

Change-Id: Iadc825f3c42b05bfc9a91462dba8a0b0068c7cd6
Signed-off-by: Vladimir Lavor <vlavor@cisco.com>