[v8] Refactor internal usage of span.startChild()

[mydea]

Apart from #10900 & #10898, we also have two legitimate usages of span.startChild(), which is in cores trace functions, and in opentelemetrys spanExporter. We need to refactor these away. In a first step, we can replace these with manually calling new SentrySpan() and creating the span like this.

### Tasks
- [ ] Refactor `startChild()` usage in core `trace` functions
- [ ] Refactor `startChild()` usage in opentelemetry `spanExporter`