pimd: Add additional IGMP stats (peak number of groups)

pimd: Add additional IGMP stats (peak number of groups)

```
exit1-debian-11# sh ip igmp statistics interface eth2
IGMP statistics
Interface            : eth2
V1 query             : 0
V2 query             : 0
V3 query             : 25
V2 leave             : 0
V1 report            : 0
V2 report            : 0
V3 report            : 34
mtrace response      : 0
mtrace request       : 0
unsupported          : 0
joins failed         : 0
joins sent           : 13
general queries sent : 2
group queries sent   : 24
peak groups          : 9
total groups         : 4
total source groups  : 1
exit1-debian-11# sh ip igmp statistics interface eth2 json
{
  "eth2":{
    "name":"eth2",
    "queryV1":0,
    "queryV2":0,
    "queryV3":25,
    "leaveV2":0,
    "reportV1":0,
    "reportV2":0,
    "reportV3":34,
    "mtraceResponse":0,
    "mtraceRequest":0,
    "unsupported":0,
    "peakGroups":9,
    "totalGroups":4,
    "totalSourceGroups":1,
    "joinsFailed":0,
    "joinsSent":13,
    "generalQueriesSent":2,
    "groupQueriesSent":24
  }
}
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>