[FR]: Disabling screen tracker and network request metrics separately in Firebase Performance

### Description

1. We want to collect as little data as possible, to make sure we are not sending data we are not using for privacy and performance reasons.
2. We are not able to disable screen tracker and network request metrics separately in Firebase Performance SDK for iOS.
3. There should be a way to disable the screen tracker and network request metrics separately, rather than having to disable everything using the `setPerformanceCollectionEnabled` method. This would allow us to better manage our data collection and limit the amount of information being collected.

### API Proposal

Either in Firebase config plist or by specifying exact metrics on Performance singleton similarly to following snippet:

```swift
Performance.sharedInstance().instrumentionGroups = [.network, .uiKit]
```

The documentation mentioning how to disable performance fetures should also mention this:

https://firebase.google.com/docs/perf-mon/disable-sdk?platform=ios

### Firebase Product(s)

Performance