New Dns debug endpoint

[ldemailly]

$ curl -s localhost:8080/fortio/rest/dns?name=debug.fortio.org | jq

Returns

{
  "Name": "debug.fortio.org",
  "IPv4": [
    "18.222.136.83",
    "192.9.142.5",
    "192.9.227.83"
  ],
  "IPv6": [
    "2600:1f16:9c6:b400:282c:a766:6cab:4e82",
    "2603:c024:c00a:d144:7cd0:4951:7106:96b8",
    "2603:c024:c00a:d144:6663:5896:7efb:fbf3"
  ]
}

Also added as a form in the main UI page

For review, only look at 6db71e0

[codecov]

Codecov Report

Patch coverage: 90.7% and no project coverage change.

Comparison is base (86740eb) 89.7% compared to head (0414e55) 89.7%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #735     +/-   ##
========================================
- Coverage    89.7%   89.7%   -0.0%     
========================================
  Files          16      16             
  Lines        4001    4025     +24     
========================================
+ Hits         3590    3610     +20     
- Misses        262     264      +2     
- Partials      149     151      +2     

Impacted Files	Coverage Δ
fgrpc/grpcrunner.go	90.7% <ø> (ø)
fgrpc/pingsrv.go	88.6% <ø> (ø)
fhttp/http_client.go	85.5% <ø> (ø)
fhttp/http_server.go	85.7% <ø> (ø)
fhttp/http_utils.go	99.1% <ø> (ø)
fhttp/httprunner.go	91.9% <ø> (ø)
periodic/periodic.go	93.7% <ø> (ø)
stats/stats.go	97.4% <ø> (ø)
rapi/restHandler.go	87.0% <84.0%> (-0.4%)	⬇️
fnet/network.go	90.4% <100.0%> (+0.1%)	⬆️

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