Adding admin command for retrieving cluster metadata

[vitarb]

Sample output:

❯ ./tctl admin cluster metadata
{
  "supportedClients": {
    "temporal-cli": "\u003c2.0.0",
    "temporal-go": "\u003c2.0.0",
    "temporal-java": "\u003c2.0.0",
    "temporal-server": "\u003c2.0.0"
  },
  "serverVersion": "1.1.0",
  "clusterId": "48477d99-5847-45e1-bdcc-29c565230259",
  "versionInfo": {
    "current": {
      "version": "1.1.0",
      "releaseTime": "2020-10-13T01:30:00Z",
      "notes": "Temporal v1.1.0"
    },
    "recommended": {
      "version": "1.1.0",
      "releaseTime": "2020-10-13T01:30:00Z",
      "notes": "Temporal v1.1.0"
    },
    "lastUpdateTime": "2020-10-15T19:48:03.506578320Z"
  },
  "clusterName": "active",
  "historyShardCount": 4
}