Experiment for now. TBD if we want to merge this.

This PR introduces a new idea how we continue SSR traces in the browser, based on <meta> tags. Previously, we'd simply continue the trace as if the SSR server-side span was the parent span of the pageload and all subsequent spans.

This has a couple of disadvantages, espsecially when subsequent browser-local root spans spans were started: All of them would count as child spans of the initial http.sever span. For some time, we had logic in the Sentry UI to artificially turn around the span relationships but this didn't work consistently within the trace view and even worse across multiple products. We'll fix this product-side by removing the reparenting logic and more clearly marking a trace as an SSR to explain why the trace looks like it does.

But what, if it doesn't have to be this way?

With this PR, we're experimenting with a different way to connect browser-local root spans with the http.server span:

• We still continue the traceId as well as of course the sampling decision and any other trace data items.
• With the important exception of the parentSpanId. Instead, we explicitly do not set a parent span id, making the browser-local root spans become actual root spans in the trace.
• We attach a span link to the browser.request span that connects this span to the SSR http.server span. This should be enough information for the product, to eventually still show the connection in the trace view.
• Importantly, this means that the product will show all browser spans as root spans now and, due to timing, the "trace root" will be the pageload span.

Drawbacks:

• The SSR http.server span is still not a child of the browser.request span. We can't solve this reliably on a technical level (requires major hacks, uncertainties and SDK changes across all our SDKs). The product can still artificially reparent these spans, though only this time, it knows exactly what to do and doesn't have to rely on heuristics.
• We now always emit traces with multiple root spans in case of SSR.