[Discover] Show unmapped fields in field list and docs regardless of data fetching

[wylieconlon]

This keeps the existing behavior of Kibana consistent across versions, because the existing behavior is to show unmapped fields. To reproduce, create a mapping without dynamic fields:

PUT mapping-test
{
  "mappings": {
    "dynamic": false
  }
}

And then insert any document:

PUT mapping-test/_doc/1
{
  "unmapped": true
}

and look in Discover to see the new field.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[kertal]

Closing this because it works like this