add XML users grants example

docker-compose.yaml

services:
  clickhouse:
    image: ${CLICKHOUSE_IMAGE:-clickhouse/clickhouse-server}:${CLICKHOUSE_VERSION:-latest}
    volumes:
      - ./xml_user_grants.xml:/etc/clickhouse-server/users.d/xml_user_grants.xml

xml_user_grants.xml

<clickhouse>
    <users>
        <another>
            <password/>
            <networks>
                <ip>::/0</ip>
            </networks>
            <profile>default</profile>
            <quota>default</quota>
            <grants>
                <query>GRANT SHOW ON *.*</query>
                <query>GRANT CREATE ON *.* WITH GRANT OPTION</query>
                <query>GRANT SELECT ON system.*</query>
                <query>REVOKE CREATE ON system.*</query>
            </grants>
        </another>
        <admin_user>
            <password/>
            <networks>
                <ip>::/0</ip>
            </networks>
            <profile>default</profile>
            <quota>default</quota>
            <grants>
                <query>GRANT admin_role</query>
            </grants>
        </admin_user>
    </users>
    <roles>
        <test_role>
            <grants>
                <query>GRANT SHOW ON *.*</query>
                <query>REVOKE SHOW ON system.*</query>
                <query>GRANT CREATE ON *.* WITH GRANT OPTION</query>
            </grants>
        </test_role>
        <admin_role>
            <grants>
                <query>GRANT ALL ON *.* WITH GRANT OPTION</query>
            </grants>
        </admin_role>
    </roles>
</clickhouse>