Deprecate passing a worker to PipelineRunner.run()

Register the worker with PipelineRunner.add_workers() before calling
run() instead. The worker argument still works but now emits a
DeprecationWarning and will be removed in a future release.

Update the runner docstrings, the run_test() helper, and all examples
(including the asyncio.gather() forms) to use the new pattern.

changelog/4493.deprecated.md

@@ -0,0 +1 @@
+- Passing a worker to `PipelineRunner.run()` is deprecated. Register the worker with `PipelineRunner.add_workers()` before calling `run()` instead; `run()` now blocks until `end()` / `cancel()` is called rather than until the passed worker finishes. The `worker` argument still works but emits a `DeprecationWarning` and will be removed in a future release.