Add span stacks

[bwoebi]

Description

We need some form of span stacks for fibers, to be able to attach active spans and their ancestors to a specific fiber and switch these out along any fiber switches.

Starting out with designing tests to figure out the semantics.

Readiness checklist

• [ ] (only for Members) Changelog has been added to the release document.
• [ ] Tests added for this feature/bug.

Reviewer checklist

• [ ] Appropriate labels assigned.
• [ ] Milestone is set.
• [ ] Changelog has been added to the release document. For community contributors the reviewer is in charge of this task.