[SPARK-17932][SQL][FOLLOWUP] Change statement `SHOW TABLES EXTENDED` to `SHOW TABLE EXTENDED`

## What changes were proposed in this pull request?

Change the statement `SHOW TABLES [EXTENDED] [(IN|FROM) database_name] [[LIKE] 'identifier_with_wildcards'] [PARTITION(partition_spec)]` to the following statements:

- SHOW TABLES [(IN|FROM) database_name] [[LIKE] 'identifier_with_wildcards']
- SHOW TABLE EXTENDED [(IN|FROM) database_name] LIKE 'identifier_with_wildcards' [PARTITION(partition_spec)]

After this change, the statements `SHOW TABLE/SHOW TABLES` have the same syntax with that HIVE has.

## How was this patch tested?
Modified the test sql file `show-tables.sql`;
Modified the test suite `DDLSuite`.

Author: jiangxingbo <jiangxb1987@gmail.com>

Closes #16262 from jiangxb1987/show-table-extended.