feature: include CLS sources on ui.webvitals.cls span

### Problem Statement

The browser SDK provides a list of the elements that were shifted by a CLS event, we should include this as an attribute on spans, as it would help people root cause:

See: https://developer.mozilla.org/en-US/docs/Web/API/LayoutShift/sources

### Solution Brainstorm

This value should be available from the web vitals SDK, we just need to add it as an attribute.