[SPARK-14184][SQL] Support native execution of SHOW DATABASE command and fix...

[SPARK-14184][SQL] Support native execution of SHOW DATABASE command and fix SHOW TABLE to use table identifier pattern

## What changes were proposed in this pull request?

This PR addresses the following

1. Supports native execution of SHOW DATABASES command
2. Fixes SHOW TABLES to apply the identifier_with_wildcards pattern if supplied.

SHOW TABLE syntax
```
SHOW TABLES [IN database_name] ['identifier_with_wildcards'];
```
SHOW DATABASES syntax
```
SHOW (DATABASES|SCHEMAS) [LIKE 'identifier_with_wildcards'];
```

## How was this patch tested?
Tests added in SQLQuerySuite (both hive and sql contexts) and DDLCommandSuite

Note: Since the table name pattern was not working , tests are added in both SQLQuerySuite to
verify the application of the table pattern.

Author: Dilip Biswal <dbiswal@us.ibm.com>

Closes #11991 from dilipbiswal/dkb_show_database.