Added _block_number column for MergeTree tables

Why it matters

The _block_number column provides the block number within MergeTree tables, enabling users to identify and reference the order of blocks in queries. This feature helps in debugging, auditing, and analyzing data ingestion or block processing by providing visibility into the internal block structure.

How to use it

Users can simply query the _block_number column in their SELECT statements on MergeTree tables to retrieve the block number assigned internally. No special setup or configuration is needed as the column is automatically available in all MergeTree tables once updated.