Row count estimate

[fishmonger45]

Adds a row count estimate to the metadata plugin for query service. Although it uses a custom gql type rather than a generic json object.
Implements: #734
Example:

query {
  _metadata {
    rowCountEstimate {
      table,
      estimate
    }
  }
}

{
  "data": {
    "_metadata": {
      "rowCountEstimate": [
        {
          "table": "starter_entities",
          "estimate": 195220
        },
        {
          "table": "_metadata",
          "estimate": 9
        }
      ]
    }
  }
}

[ianhe8x]

There wasn't any error before?

[fishmonger45]

Nope. It gets cast to string fine but it was really [string]. Can't trust the types with a whole lot of any being used here, but the plugin needs it so no avoiding.

[fishmonger45]

Seems my chaintypes error pull that has been merged is causing problems with the CI. I'll fix this by either removing the process exit and moving it to the validate or mocking process exit in that test.

[ianhe8x]

@wqsz7xn I can merge this one and you fix it in another PR.