Add function for extracting the active span id from within a host function

Via the observability workshop: if an instrumented module calls into a host function, we might lose span association if the host function makes any external calls. We may want to add a function to get the current active span from `CurrentPlugin`.

Start by tracing how CurrentPlugin represents or exposes the active span when an instrumented module calls a host function. Check the existing instrumentation and host-call paths, then define the function's expected behavior for external calls and verify that span association is preserved.

Written by the indexing model from the issue text.