Show fields which not indexed when create feature

[ylwu-amzn]

When create feature, we will show fields in dropdown list to let user choose. But if some field is not indexed, we will ignore it.

For example, if the index has such mapping

{
  "mappings": {
    "properties": {
      "is_error": {
        "type": "boolean"
      },
      "message": {
        "type": "text"
      },
      "timestamp": {
        "type": "date",
        "format": "strict_date_time||epoch_millis"
      },
      "type": {
        "type": "keyword"
      },
      "value1": {
        "type": "double"
      },
      "value2": {
        "properties": {
          "sum": {
            "type": "float"
          }
        }
      },
      "value3": {
        "properties": {
          "sum": {
            "type": "float",
            "index": false
          }
        }
      }
    }
  }
}

Then user can only see "value1", "value2.sum" in field dropdown list when create feature, can't see value3.sum. But actually user can still query field value3.sum, so we need to show this field as well.

[ylwu-amzn]

Related issue #284
Issue introduced in AD Kibana initial PR 0e5ace2#diff-b264d7ef2df743a48d3b5c0f306d37fa0c9b984b8d469c2b6e46d43fbbe754f7R20

We should remove index checking in shouldSkip method: https://github.com/opendistro-for-elasticsearch/anomaly-detection-kibana-plugin/blame/main/public/redux/reducers/mapper.ts#L20