Add Firebase to iOS app extension

### Step 0: Are you in the right place?

* For issues or feature requests related to __the code in this repository__
file a Github issue.
* If this is a __feature request__ please use the Feature Request template.
* For general technical questions, post a question on [StackOverflow](http://stackoverflow.com/)
with the `firebase` tag.
* For general (non-iOS) Firebase discussion, use the [firebase-talk](https://groups.google.com/forum/#!forum/firebase-talk)
google group.
* For backend issues, console issues, and other non-SDK help that does not fall under one
of the above categories, reach out to
[Firebase Support](https://firebase.google.com/support/).
* Once you've read this section and determined that your issue is appropriate for
this repository, please delete this section.

### [REQUIRED] Step 1: Describe your environment

* Xcode version: 11.7
* Firebase SDK version: 6.7.1
* Firebase Component: Core
* Component version: 6.7.1
* Installation method: CocoaPods

### [REQUIRED] Step 2: Describe the problem
The iOS application extension is not connecting with Firebase servers.

#### Steps to reproduce:

What happened? How can we make the problem occur?
* Create a new app into the firebase
* Follow the steps to integrate the Firebase into the iOS app extension

If you have a downloadable sample project that reproduces the bug you're reporting, you will
likely receive a faster response on your issue.

#### Relevant Code:

```
class PacketTunnelProvider: NEPacketTunnelProvider {

override func startTunnel(options: [String : NSObject]?, completionHandler: @escaping (Error?) -> Void) {
FirebaseApp.configure()
}
}
```