Add `activate()` and `deactivate()` span API

[sentrivana]

Right now, starting and finishing a span without the context manager is clunky, having to call __enter__() and __exit__(). We should consider porting activate() and deactivate() from potel-base.

Note: naming TBD, as well as if we actually want this feature. Also consider how this interacts with the new span first API.

Reference PR from potel-base: #4447

[linear]

PY-1890