Extend support to Elasticsearch nested field queries

[fernandoprocy]

Hey, everyone,

In my first PR I mentioned this other issue with Elasticsearch support. Luckily I got some free time and could work on that this weekend.

Currently, Redash doesn't parse nested field queries results properly.

As illustration, there is the logs sample dataset from Elasticsearch, in which there are a few nested structs, like machine and geo.

logs
├── machine
│   ├── os
│   └── ram
├── geo
│   └── coordinates
│       ├── lat
│       ├── lon
│       └── ...

Currently, Redash doesn't parse the json result and send it as is, even if you query for a single nested field.

This PR adds support to that, parsing the dict using a dict flatten generic function. If the user queries the parent structure, it will return every nested field. If they only query the nested field, only the nested field will get returned.

Any criticism will be welcome. Thanks!

[arikfr]

Thank you for your effort on this and apologies that it wasn't merged eventually. Due to the state of the current implementation and the inability to properly test new changes, we decided to commit to making a new Elasticsearch query runner (#4293) and will address this and the other issues there.