feat: add schema_extraction method and example

[Sdddell]

Custom parameters:

params = {
    "md_info": [],  # list: empty or parsed pdf markdown
    "db_schema": None,  # None or parsed db schema list
    "sample_mapping": None,  # None or parsed sample mapping dict
    "info_table_names": [],  # list: empty or parsed info table names list
    "target_table": None,  # None or target table name string
    "required_keys": [],  # list: empty or required keys list
}