Top hits aggregation - support multitype type hits

[llRandom]

Describe the feature:
Current version doesn't allow to get deserialized documents from TopHitsAggregate aggregation object if they have different types. An attempt to use an object as a generic type leads to an exception. The only workaround I've found is to use Newtonsoft.Json.Linq.JObject type for getting hits and then cast hits using ToObject(targetType) method:

var doss = topHitsAgg.Hits<Newtonsoft.Json.Linq.JObject>()
    .Select(hit =>
    {
        var type = GetType(hit.Type);
        return hit.Source.ToObject(type);
    });

[Mpdreamz]

We have a special ILazyDocument type that you can use to defer the deserialization of your document type.

Then you can call .As<MyObject>() on these to materialize them.

[llRandom]

Thank you, that might work as a workaround instead of Newtonsoft.Json.Linq.JObject type but this interface doesn't allow to specify cast type at runtime (I can't pass required deserialization type as an argument). Besides, it looks odd to me that approaches for root hits and hits from top aggregation are so different.

[Mpdreamz]

@llRandom while I agree this would be a super handy feature, in NEST 5.x it would be incredibly hard to introduce without backwards compatible breaking changes. Closing this as Elasticsearch 6.x no longer supports multiple types this should be ideally handled by the serializer itself.

In NEST 6.x and up it will be possible to configure a serializer just for _source's and field values so the responsibility will shift.