PipelineTask: add external pipeline task setup files

changelog/3397.added.md

@@ -0,0 +1,6 @@
+- Added support for setting up a pipeline task from external files. You can now register custom pipeline task setup files by setting the `PIPECAT_SETUP_FILES` environment variable. This variable should contain a colon-separated list of Python files (e.g. `export PIPECAT_SETUP_FILES="setup1.py:setup.py:..."`). Each file must define a function with the following signature:
+
+  ```python
+  async def setup_pipeline_task(task: PipelineTask):
+      ...
+  ```

changelog/3397.deprecated.md

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+- Loading external observers from files is deprecated, use the new pipeline task setup files and `PIPECAT_SETUP_FILES` environment variable instead.