Add Python Runner service and documentation

- Introduced a new internal utility service, python-runner, in docker-compose.yml to execute custom Python scripts within the Docker network.
- Added detailed documentation in n8n-installer-developer-guide.md and README.md on how to enable and use the python-runner service.
- Created main.py and requirements.txt files for user-defined Python code and dependencies.
- Updated scripts to include python-runner in the service selection wizard and final report for improved user guidance.

python-runner/main.py

@@ -0,0 +1,3 @@
+print("Python runner is up!")
+
+