[FR]: Add `- (void)setNumberOfStackFramesToSkipForNotFatalErrors:(NSUInteger)frames` to Crashlytics

### Description

1. When reporting non-fatal error to Crashlytics we use our helper class to enable testing
2. This cause errors to be shown with same title on our dashboard.
See https://github.com/firebase/firebase-ios-sdk/issues/11474
4. Proposal:
1. add method to set number of stack trace frames to be skipped, e.g. `- (void)setNumberOfStackFramesToSkipForNotFatalErrors:(NSUInteger)frames`
2. remove given number of frames from error stack trace when reporting

### API Proposal

```
@interface FIRCrashlytics
- (void)setNumberOfStackFramesToSkipForNotFatalErrors:(NSUInteger)frames;
@end
```

### Firebase Product(s)

Crashlytics