add function calling LLM base class (#12222)

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

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

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

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