feat(node-core): Add POtel server-side span streaming implementation (#19741)

This PR adds a server-side span streaming implementation, for now scoped
to POtel SDKs. However we can reuse some stuff from this PR to very
easily enable span streaming on Cloudflare, Vercel Edge and other
OTel-less platforms.

Main changes:

- added `spanStreamingIntegration` to `@sentry/core`: This orchestrates
the span streaming life cycle via the client and the span buffer. It's
very similar to the already existing `spanStreamingIntegration` in
browser but doesn't expose some of the behaviour that we need only in
browser.
- adjusted `SentrySpanProcessor` to emit the right client hooks instead
of passing the span to the `SpanExporter`.
- adjusted the SDKs' default integrations to include
`spanStreamingIntegration` when users set `traceLifecycle: 'stream'` in
their SDK init.

Rest are tests and small refactors. I'll follow up with Node integration
tests once this is merged to avoid bloating this PR further.

ref #17836