MetricKit Crash Fallback

### Description

If SentryCrash cannot capture a crash report, the SDK can use MetricKit crash data as a fallback. This would increase the reliability of our SDK.

It can happen that SentryCrash can't capture and report some types of crashes MetricKit can. The watchdog termination integration currently reports such crashes as watchdog terminations. Before doing that, the SDK could wait for MetricKit to report a crash. If it does, the SDK creates a normal crash event and sends it to Sentry, instead of a watchdog termination event. Since iOS 15, this happens almost immediately when starting the app again after a crash.

The goal is not to replace SentryCrash with MetricKit, as with SentryCrash the SDK can store the app's current state to disk, which is impossible with MetricKit.