[wip] add function calling agent (#12196)

docs/docs/examples/agent/mistral_agent.ipynb

docs/docs/examples/agent/react_agent_with_query_engine.ipynb

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

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

...-index-core/llama_index/core/agent/function_calling/BUILD

...-core/llama_index/core/agent/function_calling/__init__.py

...ndex-core/llama_index/core/agent/function_calling/base.py

...ndex-core/llama_index/core/agent/function_calling/step.py

llama-index-core/llama_index/core/llms/llm.py

llama-index-core/llama_index/core/tools/calling.py

...a-index-llms-mistralai/llama_index/llms/mistralai/base.py