User can now specify customized content column and metadata JSON column in replace of the default "page_content" and "langchain_metadata" column.

Summary of changes:

• MySQLEngine
  • init_document_table can create column with non-default name content_column and metadata_json_column, deprecate store_metadata.
• MySQLLoader
  • Add metadata_json_column for loading document.metadata from non-default JSON metadata field.
  • raise ValueError if specified columns cannot be found in table.
• MySQLSaver
  • Add content_column for saving document.page_content into non-default content field.
  • Add metadata_json_column for saving document.metadata into non-default JSON metadata field.
  • Support deleting document with non-default content and JSON metadata field.
• test_mysql_loader.py
  • test_save_doc_with_customized_metadata will use non-default content and JSON metadata column.
  • test_delete_doc_with_customized_metadata will use non-default content and JSON metadata column.