[SPARK-14127][SQL] Makes 'DESC [EXTENDED|FORMATTED] <table>' support data source tables

## What changes were proposed in this pull request?

This is a follow-up of PR #12844. It makes the newly updated `DescribeTableCommand` to support data sources tables.

## How was this patch tested?

A test case is added to check `DESC [EXTENDED | FORMATTED] <table>` output.

Author: Cheng Lian <lian@databricks.com>

Closes #12934 from liancheng/spark-14127-desc-table-follow-up.