Support show variables like syntax

**Describe the feature you'd like**

Now mysql supports "show variables like" to show some variables instead of all variables. Refer to https://dev.mysql.com/doc/refman/8.0/en/show-variables.html .

But it does not support in OpenMLDB SQL.


**Additional context**

We have support `like` udf in other SQLs and may use in `show variables`.

The issue is about extending the 'SHOW VARIABLES' command in OpenMLDB to support a 'LIKE' clause, similar to MySQL. Look for the SQL parser and command executor code that handles 'SHOW VARIABLES'. Check existing 'LIKE' UDF implementations to understand the pattern matching logic. Test the change by running the modified SQL command in the OpenMLDB CLI to verify it filters variables correctly.

Written by the indexing model from the issue text.