Aggregate data in Populate Virtual

**Do you want to request a *feature* or report a *bug*?**
*feature*

**What is the current behavior?**
Populate virtuals only allow for a simple query with a few conditions, but not for an aggregate query

**What is the expected behavior?**
It would be great to be able to fetch aggregate data in a populate virtual.
For example, if I have a Hotel schema:
```
var HotelSchema = new Schema({
name: {
type: String,
required: true,
}
});
```
And a hotel has many rooms:
```
var RoomSchema = new Schema({
roomNumber: {
type: String,
required: true,
},
price: {
type: Number,
required: true,
}
});
```

If I wanted to get the lowest priced room for a hotel (As you might often see on a booking website: from $109.99/night), I would need to do an aggregate query on `Hotel` model:
```
Hotel.aggregate([
{
$lookup: { /* room stuff */ }
},
{
$addFields: {
minPrice: { $min: 'rooms.price' },
},
},
]);
```
But the problem with this is you would lose a lot of nice mongoose functionality for Hotels, just because you want to add the single field (ex: no virtuals, no populating other collections without using $lookup).

So Ideally it would be nice if you could define this aggregate calculation somehow in a virtual, and simply call `Hotel.find({}).populate('minPrice')`

**What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.**
Node: 10.16.0
Mongoose: 5.7.11
MongoDB: 4.0.6

The issue names no repository files or tests. Start by reviewing Mongoose populate virtuals and aggregation APIs, then define how an aggregate calculation would be configured and invoked through Hotel.find({}).populate('minPrice'). Done means aggregate data such as a hotel's lowest room price can be exposed while retaining the stated Mongoose functionality.

Written by the indexing model from the issue text.