Zabbix

........S. [ZBXNEXT-6773] added new aggregation functions 'count', 'exists_item', 'item_filter'

Merge in ZBX/zabbix from feature/ZBXNEXT-6773-5.5 to master

* commit 'e1d0759ca1b6d4bb0dcfc55ed9763a1950aadc93':
  ........S. [ZBXNEXT-6773] refactored
  ........S. [ZBXNEXT-6773] improved error messages when validating math function arguments
  ..F....... [ZBXNEXT-6773] fixed unit tests
  ..F....... [ZBXNEXT-6773] added new aggregation functions count, exists_foreach and item_count
  ........S. [ZBXNEXT-6773] added cmoka tests for count, small code style fixes
  ........S. [ZBXNEXT-6773] added new aggregation functions 'count', 'exists_item', 'item_filter'
  ..F....... [ZBXNEXT-6773] implemented "exists_foreach" function of calculated item formula