Implement own SQLDatabase class, remove langchain (#7929)

docs/api_reference/struct_store.rst

docs/examples/data_connectors/DatabaseReaderDemo.ipynb

.../examples/index_structs/struct_indices/SQLIndexDemo.ipynb

llama_index/__init__.py

llama_index/bridge/langchain.py

llama_index/indices/common/struct_store/base.py

llama_index/indices/common/struct_store/sql.py

llama_index/indices/struct_store/container_builder.py

llama_index/indices/struct_store/sql.py

llama_index/indices/struct_store/sql_query.py

llama_index/objects/table_node_mapping.py

llama_index/readers/database.py

llama_index/utilities/sql_wrapper.py

requirements.txt

tests/indices/struct_store/test_base.py

tests/indices/struct_store/test_sql_query.py

tests/utilities/test_sql_wrapper.py