FR: improve Crashlytics support for Kotlin Native

[rickclephas]

Feature proposal

• Firebase Component: Crashlytics

Kotlin handles exceptions quite differently from Swift/ObjC.
We can already map a Kotlin Exception to a FIRExceptionModel,
but there are currently 2 missing APIs to fully support Kotlin Native:

1. FIRExceptionModel doesn't have an API to store the cause of an exception.
2. there is currently no way to manually record a fatal exception.
   In case an unhandled Kotlin exception reaches ObjC, Kotlin will terminate the program. This results in a SIGABRT crash in Crashlytics. There is a hook so it would be great if we could manually record the fatal exception.

I think adding the following APIs would suffice:

@interface FIRExceptionModel ()

@property(nonatomic, nullable) FIRExceptionModel *cause;

@end

@interface FIRCrashlytics ()

- (void)recordFatalException:(FIRExceptionModel *)exceptionModel;

@end

I have been playing with this and IMO nr 2 should be fairly straight forward.
All the logic is there we just need to expose the API.

For nr 1 I am not sure about the impact. It seems like the iOS SDK doesn't know the caused by concept, but on the other hand the Crashlytics backend does (since it's common for Android).

Would love to make a contribution, but I wouldn't know where to start (especially for nr 1).