show database status

**Describe the feature you'd like**

Use `show database status` to summarize the overall database status.

**Additional context**

Add any other context or screenshots about the feature request here.

The issue requests a 'show database status' command. First, examine the existing SQL command parsing and execution in the codebase, likely in a parser or SQL engine module. Look for similar 'SHOW' commands to understand the pattern. Determine what 'database status' means in OpenMLDB's context—metrics like uptime, memory usage, table counts, or replication status. The implementation will involve adding a new command handler and defining the status data to collect and format.

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