[Nitro] Instrument HTTP requests

[logaretm]

Implement HTTP instrumentation for the @sentry/nitro SDK, leveraging Nitro v3's built-in TracingChannel support for h3 and srvx.

Nitro v3 introduces tracing channels (h3.request, srvx.request, srvx.middleware) powered by the diagnostics_channel module which is supported across all runtimes.

These channels emit structured events for every HTTP request and middleware invocation, which the SDK can subscribe to for creating spans automatically.

We will need to implement the following:

1. Transaction & Span Creation
2. Route Parameterization
3. Error Handling
4. Request Isolation
5. Trace Propagation (Server -> Client)

[linear]

JS-1128

[logaretm]

Started an initial PR to srvx here h3js/srvx#141

[logaretm]

Both h3 and srvx PRs got merged, unblocking the road to POC this.