WaitGroup: spawn into an explicit Fiber::ExecutionContext

[ysbaddaden]

closes #15642

[yxhuvud]

One could perhaps take a step back and consider if the marriage between wait groups and contexts should live in either of these or if it should be an entity of its own. As a concept it is getting pretty close to the concept of nurseries in the structured concurrency way of thinking.

Where are we going, in the greater path toward more sophisticated ways of setting up execution?

[ysbaddaden]

IMO the nursery-like methods (.wait(&) and .spawn(&)) are the issue. Not the ability to specify a spawn context.