[MariaDB] Add support for userstat plugin keywords

Which use case/requirement will be addressed by the proposed feature?

The MariaDB User Statistics (userstat) plugin adds 4 new keywords:

USER_STATISTICS, CLIENT_STATISTICS, INDEX_STATISTICS, and TABLE_STATISTICS

Additionally, it adds unusual variants of the SHOW and FLUSH statements which aren't covered by the MySQL/MariaDB SQL grammar yet, for example
SHOW TABLE_STATISTICS;
FLUSH USER_STATISTICS;
See also:

• https://mariadb.com/kb/en/user-statistics/
• https://mariadb.com/kb/en/show-client-statistics/
• https://mariadb.com/kb/en/show-index-statistics/
• https://mariadb.com/kb/en/show-table-statistics/
• https://mariadb.com/kb/en/show-user-statistics/

Implementation ideas (optional)

The proposed changes were already merged in the upstream project: https://github.com/antlr/grammars-v4/pull/3400