Feature: Flatten nested populate

### Feature

As of right now we can have nested populate like so :
```js
const street = await StreetModel
.findOne(
{ _id: districtId}
)
.populate({
path: 'district',
model: 'District',
populate: {
path: 'city',
model: 'City'
},
})
.lean();
```

This means in order to access city you will have to do the following `street.district.city`

### Wanted behavior
I think we should allow an optional way to **flatten** the nested population in order to allow the response to have a better structure if desired, instead of having to access nested properties so in the end we can access district like so `street.district` and city like so `street.city`. I can make a PR but from/to which branch should it be ?

### Use case
In a single nested level, this won't make much of a difference but for me, I had these kind of relations `Region->city->district->sub-district->area->street` :D, sometimes I need the street with all of the locations above it

No files, tests, or entry points are identified. Start by tracing the existing nested populate behavior and defining how an optional flattening form should represent district and city; done means nested population can opt into top-level fields without breaking the current nested structure.

Written by the indexing model from the issue text.