[FR] Add support for minimum and maximum field transforms

**Is your feature request related to a problem? Please describe.**

The _minimum_ and _maximum_ field transforms (which are available in the [REST API](https://firebase.google.com/docs/firestore/reference/rest/v1/Write#fieldtransform)) have not been implemented in the Firestore Node SDK

**Describe the solution you'd like**

I imagine the API for this would look very much like [`FieldValue.increment(...)`](https://firebase.google.com/docs/firestore/manage-data/add-data#increment_a_numeric_value), and would look something like:

```
// ...
const washingtonRef = db.collection('cities').doc('DC');

// Atomically set the value of population to the minimum of 50 or it's existing value
const res = await washingtonRef.update({
population: FieldValue.minimum(50)
});
```

**Describe alternatives you've considered**
n/a

**Additional context**
n/a