Feature: Support USE CATALOG syntax and current_catalog() function

[b41sh]

Summary

• Support USE CATALOG catalog_name to set the current catalog.
• Support current_catalog() function to get the current catalog.

After the current catalog is set, user can use the table_name directly in the query without specifying catalog_name and database_name.

for example

root@0.0.0.0:8000/default> select * from my_catalog.my_database.my_table;
root@0.0.0.0:8000/default> select current_catalog();
┌───────────────────┐
│ current_catalog() │
│       String      │
├───────────────────┤
│ default           │
└───────────────────┘

# set current catalog and current database
root@0.0.0.0:8000/default> use catalog my_catalog;
root@0.0.0.0:8000/default> use my_database;
root@0.0.0.0:8000/default> select * from my_table;

root@0.0.0.0:8000/default> select current_catalog();
┌───────────────────┐
│ current_catalog() │
│       String      │
├───────────────────┤
│ my_catalog        │
└───────────────────┘

[flashmouse]

please let me try implement this @b41sh