Nested Search

## What does this Pull Request Do?

Add Nested Search

## How should this be manually tested?

Run the tests

## Any background context you want to provide?

Example:
```python
{"filter" : { "city.name" : "New York" } }
```

## Any extra info?

@xeBuz 
Can I do ``query_string = '{"filter" : {"or" : { "city" : "New York", "lastname" : "Man" } }, "sort": { "name" : "asc" } }'``` ?