Tracing without Performance

[kahest]

See Project: getsentry/team-sdks#5
See Project Board: github.com/orgs/getsentry/projects/110

Note: implementation details/requirements for Mobile (vs. backend) SDKs TBD. PR for Java can act as reference: getsentry/sentry-java#2788

### Tasks
- [x] Verify JS implementation work with RN
- [ ] If needed re-export new helper functions to continue trace
- [x] Add documentation/reuse JS docs

[krystofwoldrich]

This task doesn't require implementation changes in the RN SDK, as the logic lives in JS SDK.