[react] Implement poor CLS web vitals

[cstavitsky]

TODO: update to ?cls=true or similar, retake screenshot. Something that will allow you to pass in multiple webvitals that are bad.

Create an opt-in scenario in the /products transaction that causes a poor CLS web vital.

• If the query parameter ?webvital=cls is used, the scenario should trigger. If the query param is not used, then poor CLS should not trigger. (refer to other usage of query parameters within the demo app, like ?se= representing a given solutions engineer.)
• Communicate in the PR an accompanying narrative of, "this is what we could do to fix the slow FID"