Allow Setting 'Instance Name' for Plugins

[ic-smiller]

The kong_plugin resource does not currently support setting the instance_name property. Please add support for this so that we can see more detail on plugins when navigating the portal.

See the Kong Admin API docs. The example provided has the following json data:

{
  "name": "rate-limiting",
  "route": "string",
  "service": "string",
  "consumer": "string",
  "instance_name": "rate-limiting-foo",
  "config": {
    "hour": 500,
    "minute": 500
  },
  "protocols": [
    "http"
  ],
  "enabled": true,
  "tags": [
    "string"
  ],
  "ordering": {
    "before": [
      "string"
    ]
  }
}

As you can see, the instance_name property is supported by the API, but not by this provider.

Thank you!