MetricKit MXDiagnosticPayload GA

[philipphofmann]

Description

Experimental initial issue: #1661

Items to complete for making the MetricKit integration GA:

### Tasks
- [x] https://github.com/getsentry/sentry-cocoa/issues/2591 required for the next task
- [x] Fix calculate image size issue
- [x] #2738
- [ ] https://github.com/getsentry/sentry-cocoa/issues/2846
- [ ] https://github.com/getsentry/sentry-cocoa/issues/2847
- [x] Validate grouping and stacktraces with customer data
- [ ] https://github.com/getsentry/sentry-docs/pull/8186
- [x] Process MetricKit payloads on a background thread.