FR: Add generic support for firestore *.collection(...) methods.

### [REQUIRED] Describe your environment

* Operating System version: OSX 10.15.7
* Browser version: Chrome for OSX 109.0.5414.119
* Firebase SDK version: JS - 9.17.1
* Firebase Product: firestore

### [REQUIRED] Describe the problem

It would be convenient to be able to use generic types when getting collection references from `firestore()` and as subcollections of docRefs.

#### Steps to reproduce:
Get a collection from firestore or a docRef.

#### Relevant Code:

```javascript
// Currently
const usersCollection = firestore().collection('users');
// usersCollection is CollectionReference

// Desired
type User = {...};
const usersCollection = firestore().collection('users');
// usersCollection is type CollectionReference
```

Same for subcollections