pw_async: Run aborted Tasks with cancelled Status

Add a pw::Status parameter to TaskFunction and adopt the
"hybrid" cancellation strategy: specify that when a Dispatcher
knows that a Task will not be running as scheduled, the
Dispatcher will still call its TaskFunction with a Status
parameter of PW_STATUS_CANCELLED to facilitate the
reclamation of Task resources by clients.

Change-Id: I00dbd4c1e52f745b82ee47e057fd6f8746c6b850
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/133154
Reviewed-by: Ben Lawson <benlawson@google.com>
Commit-Queue: Ali Saeed <saeedali@google.com>
Reviewed-by: Keir Mierle <keir@google.com>