fields added to top_hits aggregation

[semd]

The following query works and returns the fields information requested in the top_hits aggregation. However, the typescript definition for top hits aggregations (AggregationsTopHitsAggregation) is missing the fields entry.

{
  "aggregations": {
    "theAggName": {
      "top_hits": {
        "fields": [
          {
            "field": "@timestamp",
            "format": "strict_date_optional_time"
          },
          "_index"
        ]
      }
    }
  }
}

So typescript throws the following error:
Object literal may only specify known properties, but 'fields' does not exist in type 'AggregationsTopHitsAggregation'. Did you mean to write 'field'?

This PR adds the fields property to the AggregationsTopHitsAggregation type with:

fields?: (QueryDslFieldAndFormat | Field)[]

I've tested it works using the QueryDslFieldAndFormat and string types, and the array is required.

[semd]

fwiw, this comes from a Kibana PR comment elastic/kibana#178893 (comment)

[JoshMock]

@semd Thanks for this. It's best we look at the underlying specification to fix missing types like this, so that all clients benefit from the fix.

I'll see about fixing the spec soon, unless you happen to get to it before I can.

[semd]

@JoshMock That sounds good. I didn't know about this process, I just saw the type missing and jumped into the fix.
We can follow the conventional way, I already created a local type to solve the problem in Kibana.
Please let me know when the spec is fixed so I can update our side. Thanks!

[JoshMock]

I've opened elastic/elasticsearch-specification#2522 to solve this in the spec. Code generation should pick it up and include it in the next release.