[query] add show uses statement

[junnplus]

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Summary

Add show users statement.

mysql> SHOW USERS;
+------+-----------+----------+-----------+
| name | hostname  | password | auth_type |
+------+-----------+----------+-----------+
| test | localhost | password |         3 |
+------+-----------+----------+-----------+
1 row in set (0.00 sec)

Changelog

• Improvement

Related Issues

Fixes #2687

Test Plan

Unit Tests

Stateless Tests

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