feat: auto-detect custom start and stop events in workflow classes (#17865)

llama-index-core/llama_index/core/workflow/workflow.py

llama-index-core/tests/workflow/conftest.py

llama-index-core/tests/workflow/test_decorator.py

llama-index-core/tests/workflow/test_service.py

llama-index-core/tests/workflow/test_workflow.py