QueryColumnsProvider: Always quote the relation name

This fixes a bug where the reserved keyword `user` (PostgreSQL) was used
unquoted in the `MAX()` function, causing an error.

And as we produce sql queries by hand (with ipl-sql) and use strings
that were not meant to be used this way, the relation name should always
be quoted.

Author: sukhwinder33445

library/Icingadb/Data/QueryColumnsProvider.php

@@ -424,6 +424,8 @@ public function queryCustomvarConfig(Filter\Rule $filter): Select
                     );
                 }
 
+                $name = $this->getDb()->getAdapter()->quoteIdentifier($name);
+
                 $aggregates[$name] = new Expression("MAX($name)");
                 $scalarQueries[$name] = $select
                     ->resetColumns()->columns(new Expression('1'))