Add a variant of `Spawn` that returns a promise that can be awaited by another activity.
This allows us to simply implement complex cross-activity synchronization.
(necessary building block for #34740)
Also adds an inter-activity latch as a building block to test this work.
Closes#34744
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/34744 from ctiller:ninteen-ninety-nine 19074b255f
PiperOrigin-RevId: 582450643