FR: RFC custom fatal exceptions in Crashlytics

### Feature proposal

* Firebase Component: Crashlytics

Crashlytics has a nice API for reporting custom exceptions with custom stack traces, but the one issue is that there's no option to make them counted as fatal. There are a number of instances where a crash has to be reported in a custom way but it is indeed a fatal crash. For example, memory crash detection is fatal but not currently detectable in Firebase Crashlytics. Even the developer rolls their own memory crash detection, they're still limited by the fact that they can only report a non-fatal issue for it. And app-not-responding detection, such as KSCrash has, de-dupes all ANRs as the same thing even when the main thread has a different stack because the watchdog thread is the one doing the crashing there. So it'd be nice to mark the main thread as the crashing one there.

I have a high-level thought of what it might look like on a PR on my fork, https://github.com/michaeleisel/firebase-ios-sdk/pull/1/files