Always emit `http.client` v2 spans

[Lms24]

At the moment, we only sent http.client spans if a parent span is active (most likely pageload or navigation spans).

With span streaming and v2 span envelopes no longer relying on transactions and "good" transaction names, we can start to always emit http.client (+potentially other) spans. Gating this with span streaming also allows us to switch to this behaviour in a major when dropping transaction envelopes. This is important because always sending these spans likely increases span quota. => aligning with a major and calling it out in the migration docs is a necessary step here

[linear]

JS-1052