add SQL Schema Node Mapping + SQLTableRetrieverQueryEngine + obj index fixes (#6569)

CHANGELOG.md

docs/examples/agent/openai_agent_retrieval.ipynb

...ndex_structs/struct_indices/SQLIndexDemo-ManyTables.ipynb

docs/examples/query_engine/RetrieverRouterQueryEngine.ipynb

llama_index/indices/struct_store/__init__.py

llama_index/indices/struct_store/sql.py

llama_index/indices/struct_store/sql_query.py

llama_index/objects/__init__.py

llama_index/objects/base.py

llama_index/objects/base_node_mapping.py

llama_index/objects/table_node_mapping.py

llama_index/objects/tool_node_mapping.py

tests/objects/test_base.py