Support DATABASE DDLs

[jackye1995]

After catalog integration, we can support database DDLs in Flink

[fightBoxing]

Based on commit 4473c31, database DDLS is now fully supported. such as:

The current code fully supports Flink Database DDL, specifically including:

✅ CREATE DATABASE [IF NOT EXISTS]

✅ DROP DATABASE [IF EXISTS] [CASCADE | RESTRICT]

✅ SHOW DATABASES

✅ USE

✅ SHOW CURRENT DATABASE

⚠️ ALTER DATABASE — Silently ignores (reasonable behavior)