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'
```