pw_sync: add ThreadNotification and TimedThreadNotification

Adds the ThreadNotification and TimedThreadNotification facades
to pw_sync.

Also provides a generic and not-optimized backend based on
pw::sync::BinarySemaphore which is immediately used for the host
target.

Change-Id: Iac067ae10f32f9633907c05365338eb575d35097
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44020
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Ewout van Bekkum <ewout@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>