kernel: work: add k_work_queue_run()

Add the ability to block and process a work queue by invoking
`k_work_queue_run` from an existing thread. This can be particularly useful
for using the main thread to process work items, in the same vein as the
system work queue, but from a lower priority/preemptible thread.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>
Co-authored-by: Peter Johanson <peter@peterjohanson.com>