Add column index_length_column to information_schema

Why it matters

This feature addresses the need for better compatibility between ClickHouse and MySQL by providing additional metadata about table index length, which MySQL users expect to find in information_schema.tables. It helps users migrating from or integrating with MySQL to get familiar and consistent metadata information.

How to use it

Users can query the information_schema.tables system table as usual, now including the new column index_length_column that provides index length details, enabling MySQL-compatible metadata queries.