FR: Allow Performance Monitoring to be used in iOS App Extensions

### [READ] Guidelines

When filing a feature request please make sure the issue title starts with "FR:".

A good feature request ideally
* is either immediately obvious (i.e. "Add Sign in with Apple support"), or
* starts with a use case that is not achievable with the existing Firebase API and
includes an API proposal that would make the use case possible. The proposed API
change does not need to be very specific.

Once you've read this section, please delete it and fill out the rest of the template.

### Feature proposal

* Firebase Component: Performance Monitoring

Describe your use case and/or feature request here.

I have an iOS app with an Action Extension that could benefit from Performance Monitoring. It might look a little different than it does for regular apps, but I believe that this would be beneficial to many iOS developers.