Aliases for current_database and new function current_schemas for PostgreSQL compatibility

Why it matters

This feature provides PostgreSQL compatibility by adding commonly used functions current_database and current_schemas, allowing users to migrate or integrate queries more easily between ClickHouse and PostgreSQL environments.

How to use it

Use the function current_database() to get the name of the current database, and current_schemas() to retrieve the current search path schemas, just as in PostgreSQL. These functions are available by default after upgrading ClickHouse to the version including this feature.