pw_async2: Add FutureOrValue

FutureOrValue simplifies creating manual state machines where you need
to hold onto the result of a future across later suspension points after
it resolves before consuming it. Currently, you'd have to write a lot of
boilerplate and an explosion of class members. FutureOrValue stores
either an active future or its result as a variant, providing convenient
APIs for construction, polling, and value access.

Change-Id: I5a94c8d4c1b5602678b3595fefbe40f4f4d84856
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/404773