Add IORuntime.isUnderFiberContext

[armanbilge]

Follow-up to #4200. Makes more complex IOLocal/ThreadLocal interop possible in userland. cc @iRevive

Let's bikeshed (or not...) isUnderFiberContext and the property cats.effect.trackFiberContext. Not sure how I feel about "context".

[iRevive]

Here is the draft implementation: https://github.com/typelevel/otel4s/pull/870/files.
The tests pass locally.

[djspiewak]

"Context" is the only bikeshed-worthy bit of this, IMO, and I think it's not a terrible noun. "Frame" also comes to mind but is more limited and makes less sense. I'll give it some more thought today but I think this is probably good as-is.