feat(core): add util method for fetching active scenes by selector

[daymxn]

Per b/534816940,

This PR adds a common util method to FirebaseCore for finding an active scene that responds to a specific selector.

With our recent scene delegate migration work, we've ran across a few instances where we needed to iterate over the connected scenes in the application and find one that responses to a certain selector.

There's some additional nuance to how we perform this search (ie; with UISceneActivationStateForegroundInactive), which are worth documenting in a central place, and can easily be overlooked.

To avoid this issue, and to practice proper DRY principles, we expose a central method in FirebaseCore that handles this process. Pending scene delegate work PRs should also be updated to take advantage of this method after it's merged.

Tests have also been added for this new util method, and documentation articulating its usage and such have been added as well.

Example usage:

SEL selector = @selector(scene:continueUserActivity:);
UIScene *targetScene = [UIApplication
   fir_findForegroundSceneWithDelegateRespondingToSelector:selector
                                           onApplication:self.mainApplication];
if (targetScene) {
 [targetScene.delegate scene:targetScene continueUserActivity:userActivity];
}      

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