PS-8378: Allow to run EXPLAIN with lower level of privileges

[dlenev]

This patch introduces new global dynamic EXPLAIN_OBSERVER privilege which allows users which have it to run EXPLAIN statement for any DML. For users which don't have this privilege EXPLAIN for a DML statement still requires the same privileges as for the DML being explained.

The idea is that users employed by monitoring/performance analysis tools can use this privilege to get information about execution plans for problematic statements, without risk of more data exposure or undesirable data changes which occurrs when they are granted global SELECT/INSERT/UPDATE/DELETE necessary to explain any DML.