Many users have been confused and frustrated by the fact that after the automatically created page load transaction is finished, XMLHttpRequest and fetch requests are only automatically instrumented with the user explicitly wrapping these requests in a transaction.

For v8, we want to change this by utilizing single-span ingestion. By default, each request will be traced and sent to Sentry as a segment span (span.isSegment: true,). The user can turn off the behaviour with a new Sentry.browserTracingIntegration() option.

### Tasks
- [x] Create/send span envelope (already done via https://github.com/getsentry/sentry-javascript/pull/11534)
- [ ] https://github.com/getsentry/sentry-javascript/pull/11696
- [ ] https://github.com/getsentry/sentry-javascript/pull/11699
- [x] Adjust `fetch` instrumentation (https://github.com/getsentry/sentry-javascript/pull/11783)
- [x] Adjust `xhr` instrumentation (https://github.com/getsentry/sentry-javascript/pull/11783)