Send Standalone CLS spans

[Lms24]

Description

(Extracted from #12714 to split up CLS and LCP spans)

A basic CLS span implmentation needs to:

• ensure we either only send CLS as pageload span measurements (old way) OR as standalone spans
• Attach the user-agent span.data
• Attach an exclusive_time to the stand alone span
• Guard sending the span with an _experimental browserTracing option
• Ensure we also stop recording CLS once users navigate. The web vitals library does not reset the CLS score after navigations.
• Re-think 0-CLS fix
• Ensure we don't break current Replay CLS collection
• Ensure we don't break future Replay CLS collection
• Check span payload with Ingest and Performance team
• ensure standalone span also carries the attribution HTML node as attributes? @edwardgou-sentry (when you're back) does this make sense? I just wonder how we'd continue showing the nodes if the CLS span is not shown in the Sentry UI. Does it even make sense to continue sending the nodes then? We can also set the span name to the html tree as string like we do for INP.
• Wait for Ingest changes
• Test on Sentry

[Lms24]

Closing as completed; this feature is now enabled experimentally in Sentry