Add _etag virtual column for S3 table engine

Why it matters

This feature allows users to access the ETag metadata of objects stored in S3 directly through ClickHouse queries. It solves the problem of not being able to retrieve and utilize S3 object versioning or integrity information, which adds value by enabling more precise data validation, change tracking, or deduplication workflows.

How to use it

Users can query the _etag virtual column in tables created with the S3 table engine just like any other column. For example, when selecting from an S3 table, include _etag in the SELECT statement to retrieve the ETag values associated with each object.