PipelineTask: add on_pipeline_finished event

This deprecates `on_pipeline_stopped`, `on_pipeline_ended` and
`on_pipeline_cancelled`.

CHANGELOG.md

@@ -7,6 +7,24 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Added `on_pipeline_finished` event to `PipelineTask`. This event will get
+  fired when the pipeline is done running. This can be the result of a
+  `StopFrame`, `CancelFrame` or `EndFrame`.
+
+  ```python
+  @task.event_handler("on_pipeline_finished")
+  async def on_pipeline_finished(task: PipelineTask, frame: Frame):
+      ...
+  ```
+
+### Deprecated
+
+- `PipelineTask` events `on_pipeline_stopped`, `on_pipeline_ended` and
+  `on_pipeline_cancelled` are now deprecated. Use `on_pipeline_finished`
+  instead.
+
 ### Fixed
 
 - Fixed a `FastAPIWebsocketTransport` and `SmallWebRTCTransport` issue where