Add support for `$covariancePop` and `$covarianceSamp` aggregation operators

Available since MongoDB 5.0.

* https://docs.mongodb.com/manual/reference/operator/aggregation/covariancePop/
* https://docs.mongodb.com/manual/reference/operator/aggregation/covarianceSamp/

```javascript
$covariancePop: { [<numeric expression 1>,<numeric expression 2>] }
$covarianceSamp: { [<numeric expression 1>,<numeric expression 2>] }
```

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Depends on: #3711 