Add setting to query Iceberg tables as of a specific timestamp

Why it matters

This feature enables users to perform historical queries on Iceberg tables, facilitating auditing, debugging, and analysis of data at specific points in time. It solves the problem of accessing past snapshots of data without requiring manual snapshot management.

How to use it

Users can enable the feature by setting the as_of_timestamp setting when querying Iceberg tables. For example, use SET as_of_timestamp = 'YYYY-MM-DD HH:MM:SS' before running a SELECT query to retrieve data as it existed at that timestamp.