Zabbix-enums

Added
- Added Zabbix version 6.0
- Added enum form HA node object
- Added enum for SLA object
- Added enum for regexp object

Changed
- Renamed enum: maintenance.ProblemTag is now maintenance.ProblemTagOperator
- Unittests no longer use `is` for testing

Added
- Full set of enumerations for 5.2.
- Many missing enumerations for 5.0 and 5.4.
- URLs to related API objects in Zabbix documentation.
- Short descriptions of enums.

Removed
- `common` namespace - it should be not longer be used.
- `specific` namespaces - they should be not longer be used.

Changed
- The `is` operator can no longed be used to compare enumerations. This is the result of removing the `common` namespace.