Update mesh insights with DP versions

[jewertow]

Issues resolved

Fix #1327

Implementation

This feature is backward compatible and works with dataplanes that don't support versions and count them as unknown versions.

[jewertow]

I tested this feature locally with 3 different versions of Kuma DP: 1.0.1 (does not support versions), 1.0.4 and a version built from this branch (both support versions) and the resource /mesh-insights returned the following JSON:

{
 "total": 1,
 "items": [
  {
   "type": "MeshInsight",
   "name": "default",
   "creationTime": "2021-01-02T13:15:22.838483+01:00",
   "modificationTime": "2021-01-02T13:19:48.839805+01:00",
   "lastSync": "2021-01-02T12:19:48.839804Z",
   "dataplanes": {
    "total": 3,
    "online": 3
   },
   "policies": {
    "Dataplane": {
     "total": 3
    },
    "Secret": {
     "total": 1
    },
    "ServiceInsight": {
     "total": 1
    },
    "TrafficPermission": {
     "total": 1
    },
    "TrafficRoute": {
     "total": 1
    }
   },
   "dpVersions": {
    "kumaDp": {
     "1.0.0-rc2-119-g50e35395": {
      "total": 1,
      "online": 1
     },
     "1.0.4": {
      "total": 1,
      "online": 1
     },
     "unknown": {
      "total": 1,
      "online": 1
     }
    },
    "envoy": {
     "1.15.0": {
      "total": 2,
      "online": 2
     },
     "unknown": {
      "total": 1,
      "online": 1
     }
    }
   }
  }
 ],
 "next": null
}

[jakubdyszkiewicz]

looks solid! Could you write test for it in resyncer_test.go?

[jewertow]

Yes, I could try to implement test for this feature.

[jewertow]

I added a test.

[jakubdyszkiewicz]

Awesome, thanks!