add structured planning agent (#13149)

docs/docs/examples/agent/structured_planner.ipynb

docs/docs/module_guides/deploying/agents/modules.md

docs/docs/module_guides/deploying/agents/usage_pattern.md

docs/mkdocs.yml

llama-index-core/llama_index/core/agent/__init__.py

llama-index-core/llama_index/core/agent/runner/planner.py

llama-index-core/tests/agent/runner/test_planner.py

.../llama-index-llms-openai/llama_index/llms/openai/utils.py

...rations/program/llama-index-program-openai/pyproject.toml