[firebase_crashlytics] Provide mechanism to modify non-fatal error.

[xxKRASHxx]

Problem: Currently crashlytics identifies each error based on the first stackframe. So all of errors are being identified as that same error, because grabbing the stacktrace from the same method.

Solution: Allow specifying custom Name / Description / Error code for the FirebaseCrashlytics.recordError or FirebaseCrashlytics.recordFlutterError methods so they could group in a right way.