Support Describe Pinot table and schema

Pinot table conf and schema access is from restful API only right now.

It would be good to support the built-in system tables, and we can query schema in SQL style. This will be useful for users to have a similar onboarding experience that everything can come from the query console. It's also good for Pinot integrations with other systems.

Query examples for references:

SHOW TABLES

DESCRIBE TABLE myTable

SHOW COLUMNS FROM myTable

SELECT 
    TABLE_SCHEMA,
    TABLE_NAME,
    COLUMN_NAME
FROM
    INFORMATION_SCHEMA.COLUMNS
WHERE
    TABLE_SCHEMA <> 'INFORMATION_SCHEMA'