Add hook(s) in dataset return lifecycle ToCountResult/ToFindResults to allow for customization

[randylsu]

Our current need is only around aggregations, so a hook into ToAggregations at the repo level would work as well, but seems like it would be nice to have something a little bit higher up in the stack as well. An example of our exact situation is ....

Field abc.def.xyz is actually mapped into a nested field with parent abc and a filter of pivotField:def, so what the user sends in as a terms aggregation terms:abc.def.xyz actually yields an Elastic agg structure of ....

"nested_abc.def.xyz": {
      "aggs": {
        "filtered_abc.def.xyz": {
          "aggs": {
            "terms_abc.def.xyz": {
              "terms": {
                "field": "search.abc.xyz"
              }
            }
          },
          "filter": {
            "term": {
              "search.abc.pivotField": {
                "value": "def"
              }
            }
          }
        }
      },
      "nested": {
        "path": "search.abc"
      }
    }

We need to roll the actual terms agg result (terms_abc.def.xyz) up 2 levels to where the consumer would have expected it without the nested trickery. Would be great to be able to do that globally at the repo level with a simple hook.