pw_async_basic: Introduce ExecuteTask

This change introduces the virtual function
`ExecuteTask` which factors out the code to
execute a task from `ExecuteDueTasks` and
`DrainTasksQueue`.

If a derived class overrides the `PostAt` and the
`ExecuteTask` function they can acquire a resource
when the task gets posted, and can release the resource
when the task has finished executing.

Bug: 373449543
Change-Id: Ie61948157db8db18a55318e60746fd265e18f71a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/256838
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathon Reinhart <jrreinhart@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Christoph Klee <chklee@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>