[FR] Catching previously uncatchable crashes with MetricKit

### Feature proposal

* Firebase Component: Crashlytics

With iOS 14, Apple now has nice support for previously uncatchable crashes like for app hangs, low memory, etc. . Apple makes the raw data, such as the call stack, available to the application, meaning it could just be forwarded to Crashlytics to create a crash report (missing some data fields but still containing the crucial bits, the stack trace and reason for crashing). This could be handled by a generic way of letting a user tell Crashlytics a crash has occurred (https://github.com/firebase/firebase-ios-sdk/issues/5771) or some MetricKit-specific API.

Aside from making this feature request, I'd be curious to hear an estimate on when it would be done (or even prioritized). This would help myself and others weigh if it's worth forking Firebase to implement this ourselves, or if we should wait and see what Firebase releases.