to be merged after #19204

This PR adds the final big building block for span streaming functionality in the browser SDK: spanStreamingIntegation.

This integration:

• enables traceLifecycle: 'stream' if not already set by users. This allows us to avoid the double-opt-in problem we usually have in browser SDKs because we want to keep integration tree-shakeable but also support the runtime-agnostic traceLifecycle option.
  • to do this properly, I decided to introduce a new integration hook: beforeSetup. This is allows us to safely modify client options before other integrations read it. We'll need this because browserTracingIntegration needs to check for span streaming later on. Let me know what you think!
• validates that beforeSendSpan is compatible with span streaming. If not, it falls back to static tracing (transactions).
• listens to a new afterSpanEnd hook. Once called, it will capture the span and hand it off to the span buffer.
• listens to a new afterSegmentSpanEnd hook. Once called it will flush the trace from the buffer to ensure we flush out the trace as soon as possible. In browser, it's more likely that users refresh or close the tab/window before our buffer's internal flush interval triggers. We don't have to do this but I figured it would be a good trigger point.

While "final building block" sounds nice, there's still a lot of stuff to take care of in the browser. But with this in place we can also start integration-testing the browser SDKs.

ref #17836